Thinly slice an onion into slivers. Add to a pot or saucepan with 3 tablespoons of butter and caramelize over very low heat for 30-40 minutes, stirring halfway through.
Thinly slice mushrooms and add to onions. Add 2 tablespoons of olive oil and coat mushrooms. Turn heat up to medium and sauté for about 5 minutes, stirring thoroughly.
Add thinly sliced shallot, two cloves garlic (minced or thinly sliced), and chopped rosemary and parsley. Mix thoroughly and saute for another 2-3 minutes.
Slice baguette into ¼" to ½" thick slices (about 30 slices). Coat a large skillet with 2 tablespoons olive oil, then add half the baguette slices. Drizzle with 2 tablespoons of oil. Toast for 5 minutes over medium heat or until golden brown.
Flip and toast the other side for 3 additional minutes. Repeat until all the slices are toasted.
Spread a thin layer of goat cheese over crostini, then divide mushroom mixture between them (approximately 1 tablespoon per slice). Garnish with fresh parsley or rosemary. Drizzle with balsamic just before serving.
Notes
If you want to use less oil, lightly brush both sides of the slices with oil and bake in a 375 degrees F oven for 4 minutes on each side, or until golden brown.
This goat cheese mushroom bruschetta is best served warm, but it can sit out at room temperature for about an hour and still taste delicious!