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

Stir in beef broth and white vinegar. Simmer on low heat for about 5 minutes until most liquid is absorbed.

Preheat oven to 375°F (190°C). Open crescent roll dough and cut into rectangles for the mini muffin tin.

Fill each dough cup about three-quarters with the caramelized onion mixture, then top with grated Gruyère cheese.

Bake in preheated oven for 18-20 minutes until golden-brown and bubbly.

Let cool for about 10 minutes, then garnish with fresh thyme and serve warm.