Heat oil in a large saucepan over medium heat. Add onion and
garlic and cook, stirring, until soft and beginning to brown, 4 to 5
minutes. Add beer and cook until reduced slightly, about 1 minute.
Add 1 cup milk and bring to a simmer.
Meanwhile, whisk the remaining 1/2 cup milk and cornstarch in a
small bowl. Add to the pan and cook, stirring vigorously, until
bubbling and thickened, 1 to 2 minutes. Reduce heat to low, add
cheese and cook, stirring, until melted. Stir in drained tomatoes,
lime juice, salt, chili powder and cayenne (if using). Serve warm,
garnished with scallions and cilantro.
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