This is such a satisfying, versatile dish that can also be prepared using rotisserie chicken and is easy to double (just use a bigger baking dish). You can make it ahead, store in the refrigerator or freezer, then pop into the oven at the opportune moment.
Pre-heat oven to 375º F. In a medium skillet, drizzle vegetable oil into the pan and tilt to evenly coat the bottom with the oil. Heat one tortilla at a time, about 15 seconds per side, then drain on a paper towel. Repeat with all the tortillas, adding small drizzles of oil as you go.
Using an 8" x 8" square glass baking dish, start layering your casserole, starting with ½ cup salsa spread evenly on the bottom. Spread 4 tortillas over the salsa, overlapping them and slightly rising up the side of the pan about an inch. Sprinkle with half of the shrimp and crab and one can of the mild diced green chile. Drizzle 1 cup salsa over the seafood and spread evenly, then dollop 1/3 cup of the sour cream in teaspoon size drops over the salsa. Now repeat, layering four tortillas slightly overlapped, and the rest of the shrimp and crab, mild diced green chile, 1 cup salsa, 1/3 cup sour cream in dollops. Top with four more overlapping tortilla, 1 cup salsa, 1/3 cup sour cream in dollops then the shredded cheese.
Bake uncovered for 35 to 40 minutes.