Texas Spaghetti

1 lb ground beef
1 medium onion, chopped
spaghetti for 4 (cooked)
1 10 oz jar Con Queso cheese dip
2 oz Velvetta
1-16 oz can tomatoes
1-10 oz can Rotel tomatoes

Brown beef and onion, drain.  Mix with other ingredients.  Pour into 1 ½ quart casserole dish.  Bake at 350 degrees for 35 to 40 minutes.  Allow to sit 15 minutes before serving.