Step 1Beat egg substitute or eggs in bowl with wire whisk. Beat in cheese, salt and pepper. Set aside.
Step 2Lightly spray nonstick 10-inch skillet with cooking spray. Add butter and melt over low heat. Increase heat to medium and add onion and garlic. Saute until tender, 2 to 3 minutes. Add chopped spinach to skillet, stirring until spinach is totally wilted. Saute spinach about 1 minute. Pour beaten egg mixture over spinach.
Step 3As edges of egg mixture begin to set, tilt pan and lift outer edges so uncooked egg can run under. Reduce heat to low and continue cooking until frittata is golden brown on bottom and almost set on top, 4 to 5 minutes. Put large plate over top of skillet and carefully invert frittata. Slide frittata back into skillet, uncooked-side down. Cook until bottom is set, 1 to 2 minutes. Remove to serving platter. Garnish with chopped tomato.