Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1 1/2 cups cooking water, then drain the pasta and rinse under cold water; set aside.
Meanwhile, heat the olive oil in a large ovenproof skillet over high heat. Add the beef and cook, breaking up the meat, until browned, 5 minutes. Add the garlic; cook 1 minute. Reduce the heat to medium; add the crushed tomatoes, tomato paste, 1 cup of the reserved cooking water and 1/2 teaspoon salt. Let simmer while you stuff the shells, about 5 minutes, adding more cooking water if the sauce gets too thick.
Preheat the broiler. Mix the ricotta, roasted peppers, egg white, 1 tablespoon each parsley and shredded cheese, 1/2 teaspoon salt and a few grinds of pepper in a bowl. Stuff 1 tablespoon of the mixture into each shell; arrange in the skillet seam-side up. Top with the remaining 2 tablespoons shredded cheese; broil until bubbling, 5 minutes. Top with the remaining 1 tablespoon parsley.