Creating these stuffed mushrooms is a rewarding process that involves a few straightforward steps:

1. Begin by preheating your oven to 375°F (190°C). Prepare a baking sheet by lining it with parchment paper or lightly greasing it with olive oil.

2. Clean the mushrooms with a damp cloth, remove the stems, and place them on the prepared baking sheet. Drizzle with a tablespoon of olive oil, and season with salt and pepper.

3. In a skillet over medium heat, add the butter and remaining olive oil. Once the butter has melted, add the sliced onions and a pinch of salt. Cook slowly, stirring occasionally, for about 20-25 minutes, until the onions are golden and caramelized.

4. Add the minced garlic and thyme to the onions, cooking for another minute, followed by the balsamic vinegar. Stir well to incorporate and remove from heat.

5. In a bowl, combine the caramelized onions with the shredded Gruyère cheese.

6. Spoon the onion and cheese mixture into each mushroom cap, packing it gently. If desired, sprinkle breadcrumbs on top for an extra crunch.

7. Bake in the preheated oven for 15-18 minutes, or until the mushrooms are tender and the cheese is melted and bubbly.

These steps remind me of the joy of creating meals from scratch, much like my grandmother did, where each step is an act of love and patience.