To make the sauce, melt butter in a saucepan over medium heat. Add garlic, mushrooms and onion. Cook, stirring occasionally, until tender, about 3-4 minutes. Whisk in flour until lightly browned, about 1 minute.
Gradually whisk in milk, and cook, whisking constantly, until slightly thickened, about 2-3 minutes. Stir in basil, oregano and season with salt and pepper, to taste. Set aside.
Spread 1 cup mushroom sauce onto the bottom of a 9×13 baking dish; top with lasagne sheets, 1/2 of the ricotta cheese, 1/2 of the spinach, 1 cup mozzarella cheese and 1/4 cup Parmesan. Repeat with a second layer. Top with remaining noodles, mushroom sauce and cheeses.
Place into oven and bake for 40-50 minutes, or until top is browned in spots.