To prepare sauce: combine the brown sugar and cornstarch in saucepan. Stir in the 2/3 cup cold water and lemon juice. Cook quickly, stirring constantly, till thickened and bubbly. Add the apples; stir gently. Cover and simmer gently for 3 to 5 minutes or till apples are tender. Add the 2 tablespoons margarine and sausage. Stir till butter melts and sausage is hot; keep warm. To prepare omelet, beat egg whites till frothy; add water and salt. Beat egg whites till stiff peaks form. Beat egg yolks till very thick and lemon-colored. Gently fold yolks into whites. Heat the 1 tablespoon butter in 10-inch oven-proof skillet till a drop of water sizzles when dropped atop. Pour in egg mixture and spread evenly with spatula, mounding higher at sides. Cook over low heat for 8 to 10 minutes or till lightly browned. Bake in 325°F. oven for 8 to 10 minutes or till knife inserted near center comes out clean. Loosen sides of omelet with spatula. Make a shallow cut across omelet, cutting slightly off-center so that the two portions are unequal. Fold smaller portion over larger portion. Using spatula, slip omelet onto hot serving platter. Set aside 2 cup of the apple mixture. Unfold omelet and spoon remaining apple mixture across center; re-fold. Pour the reserved apple mixture on top. Serves 4.