20-minute cheesy chicken enchilada soup

Chicken second courses

2 tablespoons vegetable oil 1 cup diced white onion (about 1 small white onion) 2 cloves garlic, minced ½ cup masa harina 3 cups chicken stock 2 cups cooked, shredded chicken 1¼ cups (or 1 10-ounce can) red enchilada sauce, homemade or store-bought* 1 (14-ounce) can black beans, rinsed and drained 1 (14-ounce) can diced tomatoes, with juices (I prefer fire-roasted) 1 (4-ounce) can chopped green chiles ½ teaspoon ground cumin 8 ounces sharp cheddar cheese, freshly grated 1 teaspoon salt, or more/less to taste optional toppings: tortilla chips/strips, diced/sliced avocado, grated cheese, sour cream, chopped fresh cilantro, and/or pico de gallo