Preheat the oven to 350°F (175°C) and spread 1 cup of pasta sauce evenly across the bottom of a medium baking dish.
Bring a large pot of salted water to a boil and cook the large pasta shells according to package directions until al dente; drain and toss with 1/2 teaspoon olive oil to prevent sticking.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the ground beef, diced onion, diced red pepper, minced garlic, 1/4 tsp salt, 1/4 tsp pepper, dried parsley, and dried oregano.
Cook the beef and vegetables, breaking up the meat, until the beef is browned and onions are translucent, about 8–10 minutes; drain excess grease and let the mixture cool slightly.
In a large bowl combine the cooked beef mixture with ricotta, grated Parmesan, and 1 cup of the pasta sauce; taste and adjust seasoning if desired.
Spoon the filling into each cooked pasta shell and arrange the stuffed shells in the prepared baking dish on top of the sauce.
Top the arranged shells with the remaining pasta sauce and sprinkle shredded mozzarella evenly over the top; scatter fresh basil leaves over the cheese.
Bake uncovered in the preheated oven for about 20 minutes, until the cheese is melted and bubbly. Let rest 10 minutes before serving.