In bowl, beat eggs with skim milk, basil, oregano, thyme, salt, and pepper until frothy and light. Meanwhile, in a small pan coated with cooking spray, saute onion until limp and golden, set aside. Coat inner surface of non-stick, ovenproof, 10-inch skillet with cooking spray. On stovetop, heat to medium . Add egg mixture.
Cook, using a spatula to loosen and push down sides of frittata as with an omelette. Do not scramble. Cook until almost done, leaving egg mixture slightly runny on top. Top with reserved onion, diced peppers and sliced mushrooms. Sprinkle shredded cheese evenly on top.
Place skillet with frittata in midoven, under broiler, at least 6 inches from heat source. Remove when cheese is melted and top of frittata is golden. Slide onto hot platter or warming tray. Cut with sharp knife or pizza cutter.