Crawfish Tortellini

2 (10 ounce) packages Parmesan cheese
    tortellini, cooked as directed
1/2 cup (1 stick) butter
1 medium onion, finely chopped
2 cloves garlic, finely chopped
1/2 cup Parmesan cheese
1 cup whipping cream
2 teaspoons cornstarch, mixed with 1/4 cup cold water
1/8 cup chopped parsley
1 pound crawfish tails

Cook tortellini as directed on package. Drain and set aside. Saut