Beef and Potato Casserole from Mika

1lb red potatoes, cut in thin slices
¾ lb beef sirloin steak, cut in thin slices
1 pkg Italian salad dressing mix
1 med yellow onion, chopped
2 tbsp oil
1 pkg 10 oz frozen peas and carrots
½ cup water

Place potatoes in large microwave bowl, cover with water. Microwave 10 min. Sprinkle meat with dressing mix, toss to coat. Cook onion in hot oil on med-high heat for 5 min. Add meat. Reduce to med-low after meat is thoroughly cooked. Drain potatoes. Add to steak. Add peas, carrots, water. Cover; simmer 5 minutes or until water is absorbed.