Preheat broiler. Make sauce: In a saucepan, melt butter. Stir in flour and cook, whisking about 2 minutes. Stir in vegetable stock, a little bit at a time. Add onion and Parmesan cook until the thickness you would like. Remove onion. Set aside. In a roasting pan arrange tomatoes, eggplant, red peppers, onion and toss with garlic, olives and oil. Place pan in the oven on the top shelf for 30 minutes or until vegetables are beginning to brown. Remove from oven to a cutting board and roughly chop all the vegetables. Transfer them to a bowl. Preheat oven to 375 degrees F. Ladle about 1/3 of sauce on the bottom of a 9 by 13-inch pan. Layer 1/2 of vegetables, 1/2 cooked pasta, 1/2 of mozzarella, 1/2 pasta, 1/3 sauce, 1/2 mozzarella, 1/2 vegetables, 1/3 sauce, Parmesan. Bake, covered with foil, for approximately 35 minutes. Remove foil and bake for 10 minutes more or until bubbly.