Preheat the oven to 500°F.
Heat 1 teaspoon vegetable oil in a 10-inch oven-safe skillet over medium heat.
Add the chopped onion, minced garlic, 1 teaspoon ground cumin, and 3/4 teaspoon kosher salt and cook, stirring, until the onion is softened, about 5 minutes.
Transfer the cooked onion mixture to a large bowl and set the hot skillet aside.
To the bowl, add the 16-ounce jar enchilada sauce, 1/4 cup sour cream, and 1/4 cup water; stir until combined.
Fold in the torn corn tortillas, 4 cups shredded rotisserie chicken, and the rinsed and drained 15-ounce can black beans until everything is evenly coated with the sauce.
Spoon the mixture into the reserved hot skillet and cook over medium heat until warmed through, about 3 minutes.
Sprinkle the 3 ounces shredded cheese evenly over the top and transfer the skillet to the oven; bake until the cheese is melted and bubbling, about 5 minutes.
Let the skillet cool slightly, then top with additional sour cream, sliced jalapeños, and chopped cilantro if desired; serve directly from the skillet.