Preheat your oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Add the lasagna noodles and cook according to package instructions until al dente. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add chopped onions and minced garlic, sautéing until they turn translucent. Toss in the sliced mushrooms and cook until they soften, about 5 minutes. Finally, add the chopped spinach and cook until wilted. Season with salt and pepper. Remove from heat.

In a bowl, combine ricotta cheese, half of the mozzarella, and half of the Parmesan. Mix well and set aside.

In a 9x13-inch baking dish, spread a thin layer of cream on the bottom. Add a layer of lasagna noodles, followed by half of the vegetable mixture. Spread half of the ricotta mixture over the veggies. Repeat the layers, ending with a layer of noodles topped with the remaining cream and cheeses.

Cover the dish with foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15 minutes until golden and bubbly.

Allow the lasagna to sit for about 10 minutes before serving.