3/4 pound fresh tomatillos—husked, rinsed and quartered
1/2 cup beer, such as a pale lager
2 medium canned chipotle chiles, stemmed and seeded
2 garlic cloves, minced
1/2 tablespoon rice vinegar
1 teaspoon sugar
1 teaspoon dried oregano
2 tablespoons vegetable oil, plus more for brushing
Four 6-inch corn tortillas
1 tablespoon extra-virgin olive oil
4 skinless, boneless chicken breast halves (1 1/2 pounds)
Freshly ground pepper
2 tablespoons unsalted butter
1/4 cup heavy cream
1/4 pound mozzarella cheese, shredded (1 cup)
Preheat the oven to 200°. In a food processor, puree the tomatillos with the beer, chipotles, garlic, vinegar, sugar, oregano, 2 tablespoons of the vegetable oil and 1 teaspoon of salt.
Heat a medium skillet. Brush both sides of the tortillas with vegetable oil and cook over moderate heat, 1 at a time, turning once, until crisp, about 3 minutes. Transfer to a baking sheet and keep warm in the oven.
Heat the olive oil in a large skillet. Season the chicken with salt and pepper and cook over moderately high heat, turning once, until golden brown on both sides and just cooked through, about 15 minutes. Transfer to a plate.
Melt the butter in the large skillet. Add the tomatillo sauce and heavy cream and cook over moderately high heat, stirring frequently, until the sauce thickens, about 8 minutes. Season with salt and pepper.
Lay a tortilla on each plate. Slice the chicken breasts and arrange on the tortillas. Sprinkle with the mozzarella, spoon the sauce on top and serve.