Heat oven to 425 degrees. Grease the casserole dish. Place the chopped potatoes in the dish. Melt the margarine, and add taco seasoning. Pour the margarine mixture over the potatoes. Stir to coat well. Bake uncovered in the oven for 40 minutes. Remove the dish from the oven and stir. Return the dish to the oven and bake for about 10 minutes more. Meanwhile, brown the ground beef, add onion, and cook until soft. Add the salsa to the meat mixture (enough to mix well with the ground beef). Pour over the potatoes. Top with grated cheese (as much or little as you like). Bake uncovered about 10 minutes more until the cheese is melted.