Cook the shells: Bring a large pot of salted water to a boil. Cook the jumbo pasta shells until they are al dente according to package directions. Drain and set aside to cool slightly.
Prepare filling: In a large skillet over medium heat, cook the ground beef until it is browned, about 6–7 minutes. Drain any excess grease. Stir in the taco seasoning and 1 cup of water. Let it simmer for 3–4 minutes until thickened.
Mix in extras: Stir in the black beans, corn, and ½ cup of salsa. Cook for 2 minutes, then remove from heat.
Stuff shells: Spoon 2–3 tablespoons of the beef mixture into each cooked pasta shell.
Assemble: Spread ½ cup of salsa on the bottom of a greased 9×13 inch baking dish. Arrange the stuffed shells in a single layer. Top with the remaining salsa and sprinkle shredded cheese over the top.
Bake: Cover the dish with foil and bake at 350°F (175°C) for 20 minutes. Remove the foil and bake for another 10 minutes until the cheese is melted and bubbly.
Serve: Garnish the stuffed shells with sour cream, cilantro, and green onions before serving.