Bring a large pot of salted water to a boil. Add lasagna noodles and cook until just tender. Drain them and lay flat on a clean surface to prevent sticking.

Heat olive oil and butter in a large skillet over medium heat. Add sliced mushrooms and cook slowly. Stir often until they release moisture and turn golden brown and soft.

Add chopped onion and cook until soft and light golden. Stir in minced garlic and cook for one minute. This step adds a deep flavor to your mushroom mixture.

In a saucepan, melt butter and add flour. Cook for one minute while stirring. Slowly add milk and cream. Whisk well until the sauce thickens and becomes smooth.

Add salt, black pepper, and fresh thyme to the sauce. Stir gently and taste. Adjust seasoning if needed. The sauce should taste rich and balanced.

In a bowl, mix ricotta cheese with a little salt and pepper. This layer makes the lasagna soft and creamy. Keep it ready for layering.

Preheat oven to 375°F. Spread a little sauce on the baking dish. Add noodles, then mushroom mixture, ricotta, and sauce. Repeat layers until everything is used.

Top the final layer with mozzarella and Parmesan cheese. This will melt and create a golden crust. Make sure the top is evenly covered.

Cover the dish with foil and bake for 30 minutes. Remove foil and bake for another 15 minutes until the top is golden and bubbly.

Take the lasagna out and let it rest for 10 minutes. This helps it set properly. Slice and serve warm for the best texture and flavor.