In a saucepan cook potatoes and onion, covered, in boiling salted water for 5 minutes. Add mixed vegetables and cook 5 minutes more or till tender; drain well.
Meanwhile, for sauce, in a med saucepan stir together milk, cornstarch, bouillon granules, and pepper. Cook and stir till thickened and bubbly. Stir in cheese and mustard till cheese is melted.
Spray an 8x1 1/2" round baking dish with Pam. Spread the vegetables in the bottom of the dish. Top with the egg slices. Pour sauce over all ingredients in dish.
Bake, uncovered, in 350 deg F oven for 20 minutes. Arrange tomato slices atop. Bake, uncovered, 5 minutes more or till heated through.
Per serving: 270 calories, 16 g protein, 32 g carbohydrates, 9 g fat, 284 mg cholesterol, 591 mg sodium, 618 mg potassium.