Preheat your oven to 180°C (350°F). Gather your baking dish, skillet, grater, and small bowl for toppings.
Clean and slice the mixed mushrooms, aiming for even slices that will cook uniformly to develop a deep, earthy aroma.
Heat olive oil in a large skillet over medium heat until shimmering, then add the sliced mushrooms. Sauté, stirring occasionally, until they release water and turn a rich, deep brown—about 8 minutes—your kitchen should fill with that warm, earthy smell.
Add the finely chopped onion and minced garlic to the skillet. Cook until the onion becomes translucent and fragrant, about 3-4 minutes, stirring often. The aroma should become warm and savory, with a hint of sweetness from the onion.
Stir in the fresh thyme leaves, season with salt and pepper, and cook for another minute until the herbs release their fragrance, filling the kitchen with a fragrant herbal aroma.
Pour in the heavy cream and gently stir to coat the mushroom mixture evenly. Let it simmer for 2 minutes until slightly thickened and creamy—this creates a luxurious, velvety base.
Transfer the mushroom mixture into your prepared baking dish, spreading it out evenly. Sprinkle the grated Gruyère cheese generously over the top, ensuring coverage for that gooey, golden finish.
In the small bowl, combine panko breadcrumbs with melted butter and a pinch of smoked paprika if you like. Sprinkle this evenly over the cheese layer for a crispy, golden topping that contrasts beautifully with the tender mushrooms.
Bake in the preheated oven for 20-25 minutes, until the top is bubbling, golden brown, and fragrant. You should see the cheese melted and slightly caramelized at the edges, with a crisp topping.
Remove from the oven and let the casserole rest for 5 minutes. This helps the cheese set and makes slicing easier, revealing a bubbling, golden top and tender, earthy mushrooms inside.
Slice and serve your mushroom casserole warm, perhaps paired with crusty bread or a fresh green salad. Enjoy the comforting, layered flavors and crispy topping that make this dish a cozy favorite.
