Peel and slice onion. Place onion slices on a foil lined baking sheet that has been sprayed with olive oil spray. Wash and slice zucchini, squash and eggplant and place slices on top of onion. Drizzle with olive oil and season. Cut off top of garlic head enough to expose cloves. Place garlic on a square of foil, drizzle with olive oil, and season. Wrap foil up around garlic and place on a corner of the baking sheet with vegetables. Bake for 45 minutes.
While vegetables are roasting cook lasagna noodles. Saute mushrooms in a skillet with a drizzle of olive oil, over medium high heat, until they become golden. Set aside and add sausage to skillet. Lower heat to medium and cook until brown and crumbled. Drain on a paper towel.
Beat egg and combine with ricotta in a small bowl. Add 1/2 of the Parmesan cheese and season with salt and pepper. When vegetables are done, remove pan from oven. Use a pot holder to squeeze garlic into the bowl of ricotta. Stir ricotta to blend in garlic.
Assembly: spray a 9″ x 13″ dish with cooking oil spray and pour 1/4 of the sauce in bottom of pan. Lay down 3 lasagna noodles to cover bottom of dish. Top with a layer of yellow squash and mushrooms. Dot with half of ricotta mixture. Pour 1/4 jar of sauce over.
Add another layer of 3 lasagna noodles and top with sausage and remaining ricotta. Add remaining 3 noodles and top with eggplant slices, onion and zucchini. Lay cheese on top of vegetables. Pour remaining sauce over all and top with remaining Parmesan.
Place in refrigerator or bake at 350 for 45-60 minutes or until hot and bubbly. Let sit for 10 minutes to make cutting easier. Serves 6