Toss 8 oz sharp cheddar cheese with 1 tbsp cornstarch in a small saucepan until every shred is coated.
Grate the cheese yourself. Pre-shredded cheese is coated to stop it from clumping, and that coating makes the sauce grainy.
Add 3/4 cup evaporated milk, 2 tbsp pickled jalapeños with the brine and 1/2 tsp ground cumin. Warm over medium-low heat, stirring all the time, until the cheese melts into a smooth sauce, about 3 minutes. Don't let it boil.
Pour the sauce into a warm bowl, sprinkle with 2 scallions and serve right away with 8 oz tortilla chips and 1/2 cup salsa.
Notes
No stove? Mix everything in a microwave-safe bowl and heat it in 30-second bursts, stirring in between, for 2 to 3 minutes.Leftover sauce thickens in the fridge. Warm it up with a splash of milk.