Make creamy tomato sauce: in a large saucepan, melt butter. Saute onion until tender, about 3 minutes. Stir in tomato sauce, condensed chicken broth, Parmesan cheese, and pepper. Simmer, uncovered, for about 20 minutes, or until sauce is slightly thickened. Add cream; simmer 5 minutes longer, whisking occasionaly. Set aside.
Meanwhile, cook shells according to package directions until al dente; drain well. Place shells in a single layer on a flat surface until ready to fill.
Make filling: in a medium bowl, combine cottage cheese, peas, prosciutto, and pepper until well blended.
Carefully fill each shell with about 1/4 cup cheese mixture.
Spoon 2 cups of creamy tomato sauce in bottom of 13x9x2-inch baking dish. Arrange shells in dish. Spoon remaining sauce over shells.