Cook shells in water with the tablespoon salt until 1 minute short of the package time. Reserve 1 cup pasta water before draining.
Meanwhile, brown sausage in a large skillet for 7 to 8 minutes, breaking it into small pieces, until it reaches at least 160°F. Drain excess fat, leaving about 1 teaspoon in the skillet.
Add butter and garlic and stir over low heat for 30 seconds. Add roasted pepper strips, cream, milk and black pepper and simmer gently for 2 minutes.
Add pasta and 1/4 cup reserved water and toss for 1 to 2 minutes until tender. Remove from heat and stir in Parmesan gradually.
Loosen with more pasta water if needed and serve with the sausage and red pepper distributed among the shells.