Add lentils to a large saucepan of salted water and bring water to a boil. Cook lentils at a bare simmer until tender, about 15 to 20 minutes. While lentils are cooking, in a large skillet cook bell peppers and garlic in butter over moderate heat, stirring, until peppers are softened. Remove skillet from heat and stir in lentils, drained well, and salt and pepper to taste. Preheat oven to 375F. In a large bowl whisk together eggs and parsley. Spread lentil mixture in a buttered 6- by 8-cup gratin dish and pour egg mixture over it. Shake dish gently to distribute eggs evenly and top with Gruyere. Bake frittata in middle of oven until puffed, golden, and just set in middle, about 30 to 35 minutes. Serve hot or at room temperature, cut into wedges. Serves 6. Gourmet January 1994