For the caramelized onions:

• 2 tablespoons unsalted butter
• 2 tablespoons olive oil
• 2 large yellow onions, thinly sliced
• 1/2 teaspoon sugar
• 1/4 teaspoon salt

For the mushroom mixture:

• 16 oz cremini or button mushrooms, sliced
• 2 cloves garlic, minced
• 1 teaspoon fresh thyme or 1/2 teaspoon dried thyme
• 1/4 teaspoon black pepper
• 1/2 cup vegetable broth or beef broth
• 1/2 cup sour cream
• 1/2 cup shredded Gruyère or Swiss cheese
• 1/2 cup shredded mozzarella cheese
• 1/4 cup grated Parmesan cheese
• 1 cup crushed butter crackers or panko breadcrumbs for topping
• Chopped fresh parsley for garnish (optional)