Preheat the oven to 350 degrees. In a large skillet, add the olive oil and sauté the onions over medium heat until they start to become clear, about 4 minutes. Add the mushrooms and stir. Cook another 3-4 minutes. Add in the corn and black beans and stir together. Pour in half the jar of enchilada sauce and stir until everything is coated.
Scoop the filling onto a tortilla, about 1/3 cup at a time. Roll up the tortilla and place it in a 13-by-9-inch casserole dish. Repeat until the dish is full (there should be 6 enchiladas). Pour the remaining enchilada sauce on top. Then cover with the cheese. Put the enchiladas in the oven for 10-15 minutes or until the cheese is melted.