Pat the chicken breast fillets completely dry with a paper towel. Mix the chili powder, ground cumin, kosher salt, and black pepper in a small bowl. Rub this spice mixture evenly all over both sides of the chicken fillets.
Heat a heavy black cast iron skillet over medium-high heat and add the olive oil. Once shimmering, place the chicken in the skillet. Sear undisturbed for 5-6 minutes until a deeply golden-brown crust forms. Flip and cook for an additional 4-5 minutes until the internal temperature reaches 165°F (74°C). Remove the chicken to a plate and cover loosely to keep warm.
Lower the skillet heat to medium-low. Add the butter, scraping up the browned flavorful bits from the bottom of the pan as it melts. Sprinkle in the flour and whisk continuously for 1 minute until lightly toasted.
Slowly pour in the whole milk while whisking vigorously to prevent lumps. Allow the mixture to simmer and thicken for 2-3 minutes.
Remove the skillet completely from the heat. Gradually sprinkle in the grated Monterey Jack and white cheddar cheeses. Whisk gently until the cheeses melt completely, resulting in a thick, glossy yellow-white cheese sauce.
Return the seared chicken to the skillet, spooning the cheese sauce generously over the top so it pools beautifully around the edges of the pan. Garnish the dish evenly with the diced red tomatoes, finely chopped fresh green cilantro leaves, and thin rounds of fresh green jalapeno. Serve immediately.
