Preheat your oven to 375°F (190°C). Lightly grease the sheet pan with cooking spray or line it with parchment paper.
In a large mixing bowl, combine the shredded chicken (or black beans), black beans, corn, diced bell peppers, onion, ground cumin, chili powder, salt, and pepper. Mix well until all ingredients are combined.
Spread a thin layer of enchilada sauce evenly on the bottom of the sheet pan.
Lay out 6 corn tortillas on one half of the sheet pan, slightly overlapping them.
Spoon half of the chicken (or bean) mixture over the tortillas, then pour a third of the enchilada sauce on top. Sprinkle with 1 cup of shredded cheese.
Lay out the remaining 6 corn tortillas over the filling, repeating the layering with the remaining chicken (or bean) mixture, another third of the enchilada sauce, and sprinkle the rest of the cheese on top.
Pour the remaining enchilada sauce over the top layer of tortillas and sprinkle any additional cheese if desired.
Cover the sheet pan with aluminum foil and bake in the preheated oven for 20 minutes. Then remove the foil and bake for an additional 10 minutes or until the cheese is bubbly and golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh cilantro if desired.
Cut into squares and serve warm with sour cream on the side.