Preheat oven to 350°F.
Cook pasta shells according to package directions. Drain and rinse under cold water, set aside.
In a large pot, on medium heat, sauté onions and garlic in olive oil for 5 minutes.
Slice sausages in half lengthwise. Remove meat from the skins (discard the outer casing) and add to the onion/garlic mixture. Continue to cook, stirring often to break up the meat until sausage is completely cooked (approx. 20 minutes). Remove from heat, place mixture in a large bowl and let cool.
Combine sausage mixture with fresh parsley, sour cream, 1 cup Cheddar cheese, salt and pepper. Mix well.
Spread half the pasta sauce in a 13" x 9" oven proof casserole dish. Stuff each shell with the sausage/cheese mixture and place shells in dish open side up in a single layer. Spoon pasta sauce over the top and sprinkle with the remaining shredded cheese.
Bake the shells for 30 minutes or until the shells are heated through and cheese is melted.