Foolproof Queso Dip

Name: Foolproof Queso Dip
Prep time: 10 min
Cook time: 20 min
Total time: 30 min
Cuisine: TexMex, Mexican
Ingredients: 8 ounces pepper jack cheese, grated (use any other block cheese you like) (this is about 2 cups); 4 ounces aged cheddar cheese, grated (this is about 1 cup); 2 tsp cornstarch; 10 ounce can of diced tomatoes and chiles, do not drain (Rotel or other brand); 12 ounce can of evaporated milk (not sweetened condensed milk!); 2 slices American cheese, chopped in pieces; 1/2 tsp coarse salt, or more to taste; minced red onion; sliced green and red chiles; cilantro leaves; black pepper