In a large skillet, melt vegan butter over medium heat. Add diced onion and sauté for about 2 minutes until translucent. Stir in sliced mushrooms and cook for an additional 5 minutes until tender, seasoning with salt and pepper. Remove the mixture from heat and set aside to cool slightly.

Preheat your oven to 425°F (220°C). This ensures a hot environment for baking the Pumpkin Lasagna.

Spread a thin layer of Florentine sauce at the bottom of a casserole dish. Layer with thinly sliced pumpkin, followed by the sautéed mushroom and onion mixture. Sprinkle vegan cheese over the top. Repeat these layers until you reach the top of the dish, finishing with a layer of Florentine sauce and additional vegan cheese on top.

Cover the assembled Pumpkin Lasagna tightly with aluminum foil and place it in the preheated oven. Bake for about 45 minutes, or until the edges are bubbling and the pumpkin is tender. Remove the foil in the last 10 minutes of baking.

Once baked, let the lasagna cool for about 10 minutes before serving.