Toss the Monterey jack and Cheddar cheeses together in a small bowl.
Heat the oil in a medium cast-iron or ovenproof skillet over medium-high heat. Add the scallions, jalapeno and a pinch of salt and cook, stirring, until tender, about 3 minutes. Add the tomato and cook, stirring, until soft, about 1 minute. Add the beer and cook until all the liquid has evaporated, about 7 minutes. Remove the skillet from the heat. Add the cheese mixture to the skillet and stir once or twice to combine.
Put the skillet under the broiler and broil until the cheese is melted, bubbling and lightly browned on top, 2 to 3 minutes. Serve hot with warm tortillas or tortilla chips.
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