Quick and Easy Chili Con Queso Dip

Saute 1 small onion in 2 tablespoons butter.
Add 1 (8 oz) can cut up tomatoes;
2 tablespoons chili sauce and
1 small can chopped green chilies. 
When bubbly, add in 1 pound Velveeta cheese and

1/4 lb. cheddar cheese.

Keep it warm in a fondue pot or slow cooker.