Melt butter in a medium saucepan over medium-heat; sauté onions and garlic until soft and fragrant.
Add tomato, chilies, ground cumin, onion powder and cayenne pepper; stir and cook a few more minutes.
Add cornstarch and evaporated milk; mix well.
Bring to a bubbly boil; add cheese; cook, stirring continuously, until cheese melts and sauce is smooth.
Prepare and combine all ingredients in a mixing bowl.