1 Cut the bell pepper into thin strips. Coarsely chop the scallions. Halve (or quarter) the mushrooms.
2 In a medium saucepan, bring the broth and 1/2 teaspoon of the thyme to a boil over medium-high heat. Add the bell pepper, scallions and mushrooms. Reduce the heat to medium-low, cover and simmer, stirring occasionally, until the bell pepper is crisp-tender, about 5 minutes.
3 Uncover the vegetables, increase the heat to medium and boil off any liquid remaining in the saucepan, 3 to 5 minutes.
4 Meanwhile, in a bowl, beat the whole eggs and egg whites together with the remaining 1/2 teaspoon thyme, the salt and black pepper.
5 Preheat the broiler.
6 Meanwhile, in a medium ovenproof skillet, melt the margarine over medium-high heat until hot but not smoking. Add the vegetables and the egg mixture and spread evenly in the pan. Reduce the heat to medium, cover and cook until almost set, 7 to 10 minutes.
7 When the frittata is almost set, place it under the broiler for about 5 minutes to complete the cooking and brown the top.