In medium bowl, combine eggs, sage, salt, pepper and 1/4 cup of the cheese; beat well. Set aside.
Heat oil in medium ovenproof nonstick skillet over medium heat until hot. Add zucchini and onions; cook and stir 2 minutes or until zucchini is tender. Add egg mixture; cook 2 minutes or until egg mixture is almost set, lifting edges occasionally to allow uncooked egg mixture to flow to bottom of skillet.
Top frittata with tomato slices; sprinkle with remaining 1/4 cup cheese. Broil 4 to 6 inches from heat for 1 to 3 minutes or until top is set and begins to brown.