1. Cook spaghetti according to package directions, omitting salt. Lightly coat 10-inch deep-dish pie plate or ovenproof casserole with cooking spray. Drain pasta; arrange in pie plate or casserole, forming crust. Set aside.
2. Preheat oven to 350F. In large skillet, heat oil over low fat. Add onion, garlic and bell pepper. Cook, stirring often, until onion is softened, about 3 min. Sprinkle vegetable mixture with flour. Cook, stirring, 1 min. Remove skillet from heat. Whisk in milk, a little at a time. Return skillet to medium-low heat, stirring until sauce thickens. Remove from heat; add spinach. Cool mixture slightly; stir in herb mix and egg substitute. Pour mixture into pasta crust and sprinkle top with cheese.
3. Bake until filling is set and heated through, about 30min. Serve hot.