Preheat the oven to 375 degrees F. Stir together the beans, oregano and 1 cup cheese in a bowl. Add the chicken, season with pepper and stir until evenly combined.
Stir the salsa together with 1 cup water in a bowl, then transfer 1 cup to cover the bottom of a 13- by-9-inch baking dish. Stack the tortillas, wrap them in damp paper towels and microwave until warm and pliable, about 15 seconds.
Arrange the tortillas on a workspace, then divide the chicken filling among the tortillas (about 1/4-cup filling per tortilla). Roll the tortillas up like a cigar, then transfer them to the baking dish seam-side down, positioning the rolls so that the dish is filled evenly. Press the rolls gently with your hands so they begin to soak the sauce up. Pour the remaining salsa mixture over the rolled tortillas, then sprinkle with the remaining 2 cups cheese.
Bake the enchiladas until the cheese melts and the enchiladas are hot in the center, about 30 minutes. Serve hot with sour cream and pickled jalapenos.
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