In a large skillet over medium low heat, warm the oil. Add the onion and sauté for 5 minutes, stirring often. Add garlic and sauté another minute.
Increase heat to medium, add ground beef or poultry, chili powder, and cumin. cook the mixture until meat is crumbled and cooked through, stirring often, about 5 minutes. Add the salt, water, black beans, and corn and bring to a boil. Reduce heat and simmer/boil, stirring occasionally, for about 20 minutes, until liquid evaporates. Remove from heat and let cool for 10 minutes.
Heat the oven to 350 degrees. Using the rim of a 9 inch spring form pan as a template, trim 3 of the tortillas to the size of the pan.
Butter the bottom and sides of the pan. Press the UNTRIMMED tortilla evenly into the pan’s bottom. Spread 2 tablespoons of the sour cream over the tortilla, followed by ¼ cup of salsa. Spoon and spread a quarter of the meat mixture over the salsa, and follow with a quarter of grated cheese.
Place one trimmed tortilla on top of the cheese layer. Repeat layering until all ingredients and tortillas are used up. Finish the top with cheese.
Bake the pie until heated through, about 30 minutes. Allow it to cool for 10 minutes, then remove the rim from the spring form pan.