In a large pot, bring salted water to a boil. Gently add the jumbo pasta shells and cook according to package directions until al dente. Drain and set aside to cool slightly.
In a large skillet, heat the olive oil over medium heat. Add the ground beef or turkey, breaking it apart as it cooks. Brown the meat until fully cooked.
Stir in the taco seasoning and water, mixing well. Let it simmer for about 5 minutes until the flavors meld and the mixture thickens slightly.
Add the black beans, corn, and diced tomatoes to the meat mixture. Stir until evenly combined and heated through. Season with salt and pepper to taste.
Preheat your oven to 350°F (175°C). Spread ½ cup of sour cream in the bottom of a 9x13-inch baking dish. Stuff each cooked pasta shell with a generous amount of the taco mixture and place them in the dish, open side up.
Sprinkle the remaining shredded cheddar cheese evenly over the stuffed shells.
Cover the baking dish with foil and bake in the preheated oven for 20 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and slightly golden.
Remove from the oven and let cool for a few minutes. Garnish with fresh cilantro if desired, and serve with extra sour cream on the side.