Caramelized Onion and Spinach Grilled Cheese

A recipe for Caramelized Onion and Spinach Grilled Cheese! This sandwich is packed with caramelized onions, sautéed spinach, and melted cheese for a delicious and comforting lunch.

Foodie Extravaganza

Foodie Extravaganza is a monthly party hosted by bloggers who love food! Each month we incorporate one main ingredient or theme from The Nibble into recipes to share with you.

Lauren of From Gate to Plate chose to celebrate Grilled Cheese for today’s event! Other foods celebrated in April include Coffee Cake, Mushroom, Raisin, Soft Pretzel, Chocolate Mousse, Garlic, Pecan, Cheese Fondue, and Caramel.

Caramelized Onion and Spinach Grilled Cheese

I haven’t gotten a chance to participate in FoodieExtravaganza since November (I made Hong Kong French Toast), but I am glad to join this month with a recipe for Caramelized Onion and Spinach Grilled Cheese!

This sandwich is a great way to use up leftovers. Thin cheese slices are layered with a caramelized onion and spinach mixture between two pieces of sturdy bread.

It is then heated in a skillet until the bread is golden brown on each side and the cheese has melted.

Serve the Caramelized Onion and Spinach Grilled Cheese warm from the pan. The sandwich can be enjoyed on its own or alongside a salad for a more substantial meal.

Looking for more Grilled Cheese recipes?

Try my:

• Braaibroodjie (South African Grilled Cheese Sandwich)
• Grilled Mac and Cheese Sandwich
• Very, Very, Very Slow Grilled Cheese

Caramelized Onions

To caramelize the onion slices, cook over low to medium low heat until a deep golden color and softened. Don’t rush the process by turning the heat up. It takes time for the flavor to develop, around 30 minutes.

The amount of onions and spinach will seem like a lot at first, but the mixture will shrink down as it cooks to just enough to spread a thin layer across four sandwiches.

This step of caramelizing the onions and sautéing with the spinach can also be done a day or two in advance. Refrigerate in an airtight container until ready to assemble the grilled cheese.

A Few Grilled Cheese Tips

I used sourdough sandwich bread to hold the grilled cheese together. You want slices that are sturdy enough to not fall apart with the weight of the cheese slices and onion mixture, but also not so thick that the center doesn’t heat.

This sandwich will work with a variety of cheese slices. I personally like to use thin, pre-sliced Swiss cheese. If your cheese slices are extra thin or too small to cover the bread in a single layer, increase the amount to two slices per side.

I spread mayonnaise across the inside of one slice of bread and Dijon mustard across the inside of the other slice. If avoiding mayonnaise or mustard, simply omit.

Use butter completely softened at room temperature (about 1 tablespoon per slice) to spread over the outside of the bread without tearing it. If still cold, soften in the microwave for about 20 seconds.

If you want to serve all the sandwiches warm at once, keep the cooked ones on a baking sheet in a 200˚F (95˚C) oven as you are preparing the remainder.

Check out what everyone else made:

• Bacon Apple Mascarpone Grilled Cheese from Sew You Think You Can Cook
• Caramelized Onion Bacon Grilled Cheese from Making Miracles
• Chicken Cordon Bleu Grilled Cheese from Rae Gun Ramblings
• Crusty Griddle Baked Quesadillas with Poblano Rajas from A Day in the Life on the Farm
• Grilled Cheese in the Garden from Culinary Adventures with Camilla
• Muffuletta Grilled Cheese from Food Lust People Love
• Spicy Pear Grilled Cheese from Passion Kneaded

This recipe was originally posted in April 2015 and updated in March 2025.

Caramelized Onion and Spinach Grilled Cheese Recipe

Adapted from Simply Reem

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Caramelized Onion and Spinach Grilled Cheese

A recipe for Caramelized Onion and Spinach Grilled Cheese! This sandwich is packed with caramelized onions, sautéed spinach, and melted cheese for a delicious and comforting lunch.

Course Main

Cuisine N/A

Keyword bread, caramelized onion, grilled cheese, onion, sandwich, spinach

Prep Time 10 minutes minutes

Cook Time 40 minutes minutes

0 minutes minutes

Total Time 50 minutes minutes

Servings 4 Sandwiches

Ingredients

• 2 tablespoons (28 grams) unsalted butter plus more for spreading over the bread
• 1 tablespoon (15 milliliters) olive oil
• 2 medium onions peeled and thinly sliced
• 2 cloves garlic peeled and minced
• 1 tablespoon (15 milliliters) balsamic vinegar
• Salt and freshly ground black pepper to taste
• 3 ounces (85 grams) baby spinach leaves
• 8 slices sourdough or rustic bread
• 1/4 cup (60 grams) Dijon mustard
• 8 slices Swiss, Cheddar, Fontina, or Mozzarella cheese or 16 slices if small
• 1/4 cup (60 grams) mayonnaise

Instructions

• In a large pan, place the butter and olive oil over medium low heat.
• Once the butter is melted, add the onions and cook, stirring occasionally, until deeply golden and caramelized. Adjust heat between medium and low as needed to caramelize the onions without burning.
• Add the garlic and cook just until fragrant, 30 seconds to 1 minute.
• Stir in the balsamic vinegar and season with a large pinch of salt and pepper.
• Stir in the baby spinach and continue to cook until the leaves are wilted and any released moisture evaporates.
• Remove from heat and set aside to cool slightly.
• Place two slices of bread on a work surface.
• Brush the outside of each slice of bread with a thin layer of butter.
• Place the slices butter-side down on the work surface.
• On the inside of one slice of bread, spread a thin layer of mustard.
• Cover the mustard with a slice of cheese (or two if the slices are small).
• Cover the slice of cheese with a thin layer of the caramelized onion spinach mixture.
• Cover with another slice (or two) of cheese.
• Spread a thin layer of mayonnaise on the inside of the other slice of bread.
• Place the slice of bread, mayonnaise-side down, over the cheese to complete the sandwich.
• Repeat with the remaining slices of bread and ingredients to make four sandwiches in all.
• Place a skillet over medium-low heat.
• Gently transfer the assembled sandwich to the hot skillet. Cook, adjusting the heat between medium-low and low as needed, until the bread is golden and the cheese has melted, about 2 minutes per side.
• Repeat with the remaining sandwiches.
• Serve the Caramelized Onion and Spinach Grilled Cheese while still hot.