Whisk together cornstarch and milk in a large measuring cup until smooth. Set aside.
Melt butter in a large saucepan over medium high heat. Add onion and jalapeno. Cook (stirring frequently) until onions are translucent (about 4 to 5 minutes). Add garlic and cook until fragrant (about 30 seconds).
Give the cornstarch and milk another whisk to combine and pour into saucepan. Stir to combine. Bring to a boil, while stirring often. Cook, while stirring constantly, until thickened (about a minute).
Reduce heat to low and gradually add cheeses, a handful at a time, stirring in-between. Continue to stir until cheese is melted.
Add salt, red pepper and cumin. Stir to combine.
Stir in diced tomatoes.
Sprinkle with cilantro before serving and serve with tortilla chips.