Heat the oil in a large cast-iron skillet over medium heat. Add the chicken, season generously with salt and pepper and cook, stirring often, until light golden, about 5 minutes. Add the garlic and scallion whites and cook, stirring occasionally so the garlic doesn't burn, until the garlic and scallions are softened, about 2 minutes.
Stir in the green chiles and chili powder and cook for 1 minute. Stir in the black beans, spinach and corn. Top skillet with a lid and let the spinach wilt slightly, about 1 minute. Uncover and season with salt and pepper and stir in the salsa. Increase the heat to medium high and cook, stirring occasionally, until the spinach is completely wilted and the sauce gently simmers, 5 to 7 minutes.
Top the chicken mixture with the Cheddar, then cover and cook until the cheese is fully melted, about 3 minutes. Top with the scallion greens and any other optional garnishes such as cilantro, sour cream, more salsa or hot sauce. Serve from the skillet with warm tortillas.