1. Begin by preheating your oven to a toasty 375°F (190°C). While your oven is warming up, grab your 9×13-inch baking dish and grease it with cooking spray or a drizzle of olive oil.

2. Fill a large pot with water, season it with salt, and bring it to a rolling boil.  Once the water is bubbling away happily, add in your lasagna noodles. Once they’ve reached pasta perfection, drain them and set them aside.

3. Heat up a large skillet over a medium flame and add a splash of olive oil to get things sizzling. Toss in your finely chopped onion and let it dance in the pan until it turns translucent and fragrant, about 3–4 minutes. Sprinkle in some minced garlic and let it work for another minute.

4. Add in your sliced mushrooms, along with a sprinkle of dried thyme, salt, and pepper for seasoning. Cook them until they’re tender and any excess liquid has evaporated.

5. Add chopped spinach leaves to the skillet and cook until wilted, about 2–3 minutes. Remove from heat and set aside.

Assemble And Bake 

1. Start by ladling a thin blanket of marinara sauce onto the bottom of the dish — this will ensure each bite is packed with flavour. 
2. Place three lasagna noodles on top of the sauce for the layers to come. 
3. Spread half of the creamy ricotta cheese over the noodles, followed by a dollop of the mushroom mixture, spreading it out evenly. 

4. Sprinkle a third of the mozzarella and Parmesan cheeses over the mushrooms.

5. Repeat these layers with the remaining noodles, ricotta, mushrooms, and cheeses, finishing with a final flourish of marinara sauce on top.

6. Cover your baking dish with aluminium foil, and slide it into the preheated oven. Let it bake for a tantalizing 25 minutes

7.  After this initial bake, remove the foil and let your lasagna continue to bake for an additional 10–15 minutes, or until the cheese on top is bubbly and golden brown.