In a skillet, cook bacon until almost crisp; add onion. Cook, stirring frequently, until bacon is crisp and onion is tender. Drain. Transfer to a 1-1/2-qt. Casserole. In a mixing bowl, beat eggs, milk, biscuit mix, salt and pepper until almost smooth. Slowly pour over bacon and onion. Bake, uncovered, at 375 for 30 minutes or until a knife inserted near the center comes out clean. Sprinkle with cheese; let stand 5 minutes before cutting.