Caramelize the Onions:

• In a large skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, for 15-20 minutes, or until the onions are golden brown and caramelized. If the onions start to burn, add a splash of water and continue to cook until they soften and caramelize.

Cook the Mushrooms:

• Add the olive oil to the skillet with the onions. Add the sliced mushrooms and cook, stirring occasionally, for 7-8 minutes until the mushrooms release their moisture and become tender.
• Add the garlic, thyme, and black pepper, and cook for another minute until the garlic becomes fragrant.

Make the Sauce:

• Pour in the vegetable broth and let it simmer for 2-3 minutes, allowing the flavors to meld.
• Stir in the heavy cream and bring the mixture to a simmer. Let it cook for another 3-4 minutes until the sauce thickens slightly.

Assemble the Casserole:

• Preheat your oven to 375°F (190°C).
• Transfer the onion and mushroom mixture to a greased 9×13-inch baking dish.
• Sprinkle the shredded Gruyère cheese and Parmesan cheese over the top of the mixture, then stir to combine.

Prepare the Topping:

• In a small bowl, combine the breadcrumbs, melted butter, and grated Parmesan cheese. Mix until the breadcrumbs are well coated with butter.
• Sprinkle the breadcrumb mixture evenly over the top of the casserole.

Bake the Casserole:

• Place the casserole in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and the casserole is bubbling.

Serve:

• Remove the casserole from the oven and let it sit for a few minutes before serving.
• Garnish with chopped parsley, if desired, and serve hot.