Don Pablo's Queso Dip

1 cups onion, chopped (more or less depending on if you like onions)
3 tablespoons butter
2 cans (10 ounces each) Rotel diced tomatoes with green chilies
1 can (14.5 ounces) stewed tomatoes, chopped
2 pounds processed cheese, such as Velveeta
1 pound sharp cheddar cheese, cubed
Corn chips, Triscuits or sliced vegetables

Saute onions in butter. Add tomatoes with chilies and stewed tomatoes; cook until thick. Add processed and cubed cheeses. Cook until thick and smooth, stirring often to keep from sticking. Refrigerate overnight. Heat to eat. Serve with corn chips, Triscuits or vegetables.