Prepare Sauteed Tomato Sauce; cover and keep warm. Cook spaghetti according to package directions; drain. Return spaghetti to saucepan; cover and keep warm.

Meanwhile, preheat oven to 350 degrees F. In a large saucepan, heat olive oil over medium heat. Add onion; cook and stir about 5 minutes or until tender. Add mushrooms, zucchini, sweet pepper, and garlic. Cook and stir about 3 minutes more or just until vegetables are tender. Drain off liquid. Stir in salt, oregano, and black pepper; cool.

In a large mixing bowl, beat eggs, milk, and Parmesan cheese with an electric mixer on low speed until combined. Add cream cheese. Beat for 30 seconds.

Arrange spaghetti in the bottom of a greased 3-quart rectangular baking dish. Add vegetable mixture. Sprinkle with cheddar cheese. Pour egg mixture evenly over mixture in dish. Press mixture lightly with the back of a large spoon. Bake about 40 minutes or until mixture is set and lightly browned on edges.

To serve, cut frittata into serving-size pieces. Spoon about 1/3 cup of Sauteed Tomato Sauce onto each plate; place a serving of frittata on top of sauce. Makes 12 servings.

Sauteed Tomato Sauce: In a medium saucepan, heat 2 tablespoons butter over medium heat. Add 3/4 cup chopped onion; cook about 5 minutes or until onion is tender, stirring frequently. Add 2 cloves garlic, minced; cook and stir for 30 seconds. Carefully stir in one 28-ounce can Italian-style whole tomatoes in puree, undrained and cut up; half of a 6-ounce can (1/3 cup) tomato paste; 1/2 teaspoon sugar; 1/4 teaspoon salt; and 1/4 teaspoon ground black pepper. Bring to boiling; reduce heat. Simmer, uncovered, about 15 minutes or until mixture is of desired consistency. If desired, stir in 1 teaspoon dried oregano, crushed. Cook for 5 minutes more. Makes about 4 cups.

Make-Ahead Tip: Prepare the Sauteed Tomato Sauce as directed; cool. Transfer sauce to an airtight container and chill for up to 24 hours. To reheat, transfer the sauce to a medium saucepan. Cover and cook until heated through.