Set one half of grill to high and other half to medium-low. Thinly slice the green onions and drain the black beans. In a large bowl, combine the refried beans, black beans, chili powder, and corn. Fold in sliced green onions. Thinly slice the tomatoes. Reserve ¼ cup of the cheese.
Coat the bottom of a 10-inch cast-iron skillet with the cooking spray. Place 1 tortilla on bottom and top with ⅓ of bean mixture, ⅓ of the tomatoes, then ⅓ of the cheese. Repeat layering with 2 more tortillas, remaining beans, remaining tomatoes, and cheese. Finish with remaining tortilla and reserved ¼ cup cheese.
Place skillet on cooler side of grill. Cover and cook 15 min., until center is hot, rotating skillet once halfway through.
Meanwhile, thinly slice the romaine. Garnish the casserole with romaine.