In medium bowl, combine eggs, milk, chives, marjoram, salt and pepper; mix well. Set aside.
Heat 1 tablespoon oil in 12-inch nonstick skillet over medium-high heat until hot. Add onion, zucchini and garlic; cook and stir 1 minute. Reduce heat to medium. Add broccoli, corn and 1/4 cup water; cover and cook 4 to 6 minutes or until vegetables are crisp-tender, stirring occasionally. Remove vegetables from skillet.
Add 2 teaspoons oil to skillet. Return vegetables to skillet; pour egg mixture over vegetables. Cover loosely; cook over medium heat for 10 to 15 minutes or until center is set, lifting edges occasionally to allow uncooked egg mixture to flow to bottom of skillet.
Sprinkle frittata with cheese; arrange tomato slices on top. Cover; cook 1 to 2 minutes or until cheese is melted. If desired, garnish with additional chives or sprigs of marjoram. To serve, cut into wedges.