In a medium microwave-safe bowl combine together the onion and the garlic, spray lightly with cooking spray and microwave on HIGH for 1 minute.
Take ricotta, pimientos, Parmesan, milk, marjoram, tarragon, salt, nutmeg, and pepper and mix well with the onion and garlic. Take a 9 or 10 inch microwave-safe baking dish and spray with cooking spray, then spread 1/3 cup of the spaghetti sauce on the bottom of the dish.
Take half of the eggplant, zucchini, and summer squash and layer in the dish then top the layers with ricotta mixture. Take the remaining eggplant, zucchini, and summer squash and layer it over the ricotta mixture and then top the layers with the remaining 2/3 cup of spaghetti sauce.
Cover the dish with vented plastic wrap and microwave it on HIGH for 17 to 19 minutes or until the vegetables have become tender, making sure to rotate the dish every 6 minutes then top with mozzarella cheese and let it stand for 10 minutes before serving.