Melt butter in a medium-sized pot over medium heat. Sauté the onions and the jalapeño pepper until soft. Then add the garlic and sauté until fragrant, about 30 seconds.
Add the tomatoes, cumin, onion powder and cayenne pepper. Cook while stirring occasionally, for 2 minutes.
Mix cornstarch into the pot, then add the milk, stirring well to combine.
Bring to a gentle simmer then add the cheese, mixing well. Cook, while stirring, until cheese just melts through the milk and transforms into a smooth sauce. Take off heat immediately.
Stir in cilantro and season with salt to taste if needed, mixing well.