Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to package directions until just al dente. Drain and set aside to cool slightly.

Heat a large skillet over medium heat. Add the ground beef and cook, breaking it up with a spoon, until browned. Drain excess fat if needed.

Add the diced onion, bell peppers, and minced garlic to the skillet with the beef. Cook for 5 minutes until the vegetables soften.

Stir in the Italian seasoning, salt, and black pepper. Remove the skillet from heat and let the mixture cool for a few minutes.

Mix the ricotta cheese into the beef and vegetable mixture until evenly combined.

Spread 1 cup of marinara sauce in the bottom of a 9x13 baking dish. Stuff each shell with the beef filling and place them in the dish.

Pour the remaining marinara sauce over the stuffed shells and sprinkle the mozzarella cheese on top.

Bake at 375°F for 25 minutes until the cheese is melted and bubbly. Let rest for 5 minutes before serving.