Step 1Roast the jalapenos: Place the chiles on a rack set over a gas stove-top burner heated over high heat. Roast until the skin on all sides of each chile is charred, about 5 minutes, turning frequently. (If you have an electric or ceramic stove top, roast the chiles in the oven using the broiler setting until charred on all sides.) Wrap each pepper in plastic wrap and set aside until the peppers are cool enough to handle. Rub the plastic wrap against the skin to loosen and remove it. Do not rinse the peppers to remove the skin, as rinsing will remove flavor. Stem and seed each pepper, then dice the peppers into roughly one-fourth-inch pieces. Set aside.
Step 2In a large, heavy-bottomed saucepan, heat the oil over medium-high heat. Add the onion and cook, stirring frequently, until the onion is softened and lightly browned, 12 to 15 minutes. Stir in the garlic, then add the diced tomatoes and jalapenos. Cook until most of the liquid is evaporated, about 5 minutes, stirring frequently. Stir in the cumin.
Step 3Stir in the beer, scraping any flavorings from the base of the pan, then reduce the heat to medium-low and begin to stir in the cheese, a few cubes at a time. Continue to add more cubes as the cheese melts, stirring constantly.
Step 4Stir in one-half cup milk when most of the cheese has been added, then continue to stir until all of the cheese is melted to form the queso. Adjust the consistency of the queso with additional milk as desired. Taste and adjust the seasoning with three-fourths teaspoon salt, or as desired. This makes about 6 cups queso.