In a large skillet, drizzle olive oil over medium heat. Add the onions and cook until beginning to soften and become translucent, 10 minutes.

Add the brown sugar and decrease heat to low. Cook, stirring occasionally, until the onions caramelize and deepen in color, 10 minutes. Stir in the garlic and balsamic vinegar. Cook until the garlic becomes fragrant, 1-2 minutes. Remove the pan from heat.

In a medium bowl, combine sour cream, mayonnaise, chopped parsley, and pepper. Once the onions are cool to touch, stir them into the mixture.

Use an immersion blender to smooth the dip to desired consistency. Season with salt and refrigerate 2 hours to overnight.

Serve with vegetables or crackers.