Preheat the oven broiler.

Set the frozen asparagus in a bowl of hot water for 4 to 5 minutes, or until thawed. Remove the asparagus, and gently squeeze to remove the excess water. Place on paper towels and pat dry. Cut the asparagus into 1/2-inch pieces, and set aside.

While the asparagus is thawing, coat a 12-inch nonstick ovenproof skillet with cooking spray, add the oil, and preheat over medium-low heat. Add the onion, and cook, stirring often, for 4 to 5 minutes, or until soft and beginning to brown.

Place the egg substitute in a medium-sized bowl. Add 1 tbsp of the cheese, the salt, if desired, and the black pepper, and stir to mix well. Stir in the asparagus.

Pour the egg mixture over the sautéed onion. Allow the eggs to cook over medium-low heat for 5 to 6 minutes, or until the bottom is brown and set. Place the pan under the broiler, about 4 inches below the heat source, and cook for 3 to 4 minutes, or until the top of the frittata is cooked and light brown.

Slide the frittata onto a large platter, top with the tomato slices, and sprinkle with the remaining tbsp of cheese. Cut into wedges, and serve immediately.