Special Equipment: A pastry bag and a large closed star tip (we used size 845 tips); deep-fry thermometer.
Preheat oven to 350°. Toast cascabel until fragrant and lightly browned, about 5 minutes. Remove chiles from baking sheet, let cool.
Increase oven temperature to 450°. Roast tomatoes, fresnos and onion on a clean rimmed baking sheet until skin is browned and beginning to separate from flesh, 30–35 minutes. Transfer to a blender and add garlic, lime juice, and 2 teaspoons salt; blend until smooth. Add cascabel and blend until coarsely chopped. Let sit at room temperature until ready to serve.
Melt butter in a medium saucepan over medium heat. Stir in flour and cook until incorporated, about 1 minute. Whisk in milk and continue to cook until mixture comes to a boil and thickens, about 4 minutes. Reduce heat to low, gradually add both cheeses, and cook, stirring constantly, until cheese is completely melted and queso is smooth. If it seems too thick, stir in a little more milk. Keep warm until ready to serve.
Do Ahead: Salsa and queso can be made 2 days ahead. Cover and chill speperately. Bring salsa room temperature before serving; warm queso until smooth and creamy.
Fit pastry bag with star tip. Whisk chili powder and 1 tablespoon salt in a small bowl; set aside.
Bring milk, butter, cumin, 1¼ teaspoon salt, and ½ cup water to a simmer in a medium saucepan over medium-high heat. Using a wooden spoon, add flour and cornmeal in one go, and vigorously mix until dough comes together, about 30 seconds. Let sit in pan for 10 minutes to hydrate the cornmeal. Transfer to the bowl of a stand mixer or a large bowl.
Using a stand mixer fitted with the paddle attachment on medium-low speed, add eggs to dough, one at a time, making sure to incorporate each egg before adding the next (alternatively, stir vigorously with a wooden spoon). Dough will look broken at first; continue to beat, scraping bowl occasionally, until dough is smooth, glossy, and somewhat stretchy (pull off a small piece of dough and stretch it—it shouldn't break). Spoon dough into prepared pastry bag.
Pour oil into a large pot to come halfway up the sides. Fit pot with thermometer and heat over medium-high until thermometer registers 350°. Holding bag at an angle so tip is a few inches above surface of oil, squeeze out dough, moving the bag as you squeeze so dough is piped in a 6" length into oil. Using a paring knife, cut off dough at the tip to release into oil. Repeat process to make 4 more dough lengths. Fry churros, turning once and adjusting heat as needed to maintain oil temperature, until golden brown on all sides, 2–3 minutes per side. Transfer to a paper towel-lined baking sheet. Repeat with remaining dough.
Sprinkle warm churros with reserved chili-salt mixture mixture. Dollop salsa over warm queso and swirl to combine; serve with warm churros.