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.