Preheat the oven to 350°F (175°C) and grease or prepare a medium baking dish; spread 1 cup of pasta sauce evenly across the bottom.
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 tsp olive oil to prevent sticking.
Heat 1 tbsp 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, 1/4 tsp dried parsley, and 1/4 tsp dried oregano.
Cook the beef and vegetables, breaking up the meat, until the beef is browned and the onions are translucent, about 8–10 minutes; drain excess grease and set the mixture aside to cool slightly.
In a large bowl, combine the cooked beef mixture with 1 1/2 cups ricotta, 2 tbsp grated Parmesan, and 1 cup of the pasta sauce; taste and adjust seasoning with a little more salt and pepper if desired.
Spoon the filling into each cooked shell and arrange the stuffed shells in the prepared baking dish on top of the sauce.
Pour the remaining 1 cup of pasta sauce over the shells and sprinkle 1/2 cup shredded mozzarella on top, then scatter fresh basil leaves over the dish.
Bake uncovered at 350°F (175°C) for about 20 minutes, until the cheese is melted and the sauce is bubbly.
Let the baked shells rest for at least 10 minutes before serving to set.
