Heat oven to 350 F. In a 10- or 12-inch nonstick, ovenproof skillet, saute garlic and onion in olive oil for about 5 minutes. Add mushrooms and thyme. Cook an additional 5 minutes. Remove skillet from stove.
Place spinach in a separate saucepan. Add 1 tablespoon water. Cover and cook until just wilted. Drain spinach and let cool in a colander. Squeeze out any liquid. Chop leaves.
In a large bowl, beat together egg substitute, dill and pepper. Stir in the spinach-mushroom mixture and feta cheese.
Clean nonstick skillet. Spray liberally with cooking spray. Return skillet to stove over medium heat. When skillet is hot, pour in egg mixture.
Place in oven, uncovered. Check frittata in 10 minutes. Check every 5 minutes thereafter until center of frittata is slightly firm. Do not overcook. When frittata is done, place a large serving platter over skillet. Flip skillet over so that frittata falls onto the plate. Cut into six pieces and serve.