In a large skillet, heat olive oil and butter over medium heat. Add sliced onions, stirring occasionally for about 30 minutes until they become golden brown and caramelized.

Stir in beef broth and white vinegar, letting it simmer for 5 minutes until most of the liquid is absorbed.

Preheat your oven to 375°F (190°C). Unroll crescent roll dough, cutting it into 3x3 inch rectangles and pressing into the greased wells of a muffin tin.

Spoon the cooled onion mixture into each dough cup, filling them about three-quarters full, then top with Gruyère cheese.

Bake for 18-20 minutes until the dough is golden brown and the cheese is bubbly.

Allow bites to cool in the tin for about 10 minutes before serving warm., garnishing with fresh thyme.