In a large skillet, melt butter and olive oil over medium heat. Stir in the onion, garlic, salt, and pepper. Coat with the worcestershire sauce, thyme, bay leaf, sugar, and beef broth, then increase heat to high. Once the broth boils, reduce heat to medium. Cook, stirring occasionally until onions soften and turn a deep golden brown, 25 minutes. Remove from heat. Once cooled, stir in gruyere cheese.
Place a small bowl of water next to work surface. Put wonton wrapper on work surface. Place 1 tablespoon of onion and gruyere in center of wrapper, then wet the edges with water. Fold the wrapper and press to seal, making sure no air remains in center. Place on baking sheet lined with parchment and repeat until you run out of wrappers and filling.
In a medium skillet, drizzle about 1 teaspoon vegetable oil over medium heat, just enough to coat bottom of pan. Once heated, add a few potstickers, not overcrowding, making sure the bottom of each is coated in oil. Once the bottoms are golden, quickly pour in water until 1/3rd up the potstickers and cover. Cook until most of water has evaporated and only about 2 tablespoons remain. Uncover and continue to cook until rest of water has evaporated and bottoms are crisp. Gently remove potstickers with a spatula onto a towel lined plate. Clean skillet and repeat with desired amount of potstickers.
In a small pot, combine beef broth, worcestershire sauce, and salt. Cook over medium heat until heated through.
Serve immediately.