Grate Cheddar cheese into a bowl. Sprinkle in cornstarch and toss to coat.

Melt butter in a saucepan over medium-high heat. Add garlic and light parts of the scallions. Sprinkle in salt, chipotle, and cumin. Cook and stir until fragrant, about 2 minutes. Add green chiles and 1/2 cup tomatoes; cook and stir for 2 minutes more. Pour in evaporated milk and bring to a boil. Add the Cheddar cheese and stir just until melted; immediately remove from heat.

Stir in cilantro. Season to taste, adding cayenne if desired. Garnish with reserved green onion parts and 1 tablespoon diced tomato.