Step 1: Prepare the Baguette

1. Preheat your oven to 375°F (190°C).
2. Arrange the baguette slices on a baking sheet. Brush both sides lightly with olive oil.
3. Bake for 8–10 minutes, flipping halfway through, until golden and crispy.
4. Remove from the oven and rub each slice with the cut side of the garlic clove while still warm. Set aside.

Step 2: Caramelize the Onions

1. In a large skillet, heat the butter and 1 tablespoon of olive oil over medium heat.
2. Add the onions and a pinch of salt. Cook, stirring occasionally, for 20–25 minutes, or until the onions are golden brown and caramelized. Lower the heat if they begin to brown too quickly.
3. Stir in the balsamic vinegar during the last 2 minutes of cooking to deglaze the pan and enhance the sweetness.

Step 3: Sauté the Mushrooms

1. In the same skillet, heat the remaining olive oil over medium heat.
2. Add the mushrooms, thyme, salt, and black pepper. Cook for 8–10 minutes, stirring occasionally, until the mushrooms are golden and tender.

Step 4: Assemble the Crostini

1. Top each baguette slice with a generous spoonful of caramelized onions.
2. Add a layer of sautéed mushrooms on top of the onions.
3. Sprinkle with crumbled goat cheese or grated Parmesan, if desired.

Step 5: Garnish and Serve

1. Garnish with fresh parsley or chives for a pop of color and added flavor.
2. Serve warm or at room temperature.