Sauté the Aromatics: In a large skillet over medium heat, warm the neutral oil. Add the chopped onion and season with ½ teaspoon kosher salt. Cook, stirring occasionally, until softened (about 7 minutes). Add the jalapeño and garlic, and cook just until fragrant—about 1 minute.
Prepare the Thickener: In a small bowl, whisk together the cornstarch with ¼ cup evaporated milk until smooth. This will prevent lumps and ensure a silky texture later.
Build the Sauce: Add the paprika and the remaining evaporated milk to the skillet. Bring to a gentle boil over medium-high heat, stirring constantly. Briefly whisk the cornstarch mixture to reincorporate, then pour it into the skillet. Lower the heat and simmer, stirring frequently, until the mixture thickens (about 2 minutes).
Add Cheeses: Remove the skillet from heat. Gradually stir in the shredded sharp cheddar and Monterey Jack cheeses until fully melted and the sauce is smooth.
Add the Fresh Ingredients: Stir in the chopped tomatoes, cilantro, and lime juice. Taste and season with the remaining ½ teaspoon salt as needed.
Garnish and Serve: Top the queso with extra tomatoes and cilantro if desired. Serve immediately with plenty of tortilla chips.
