Cook sausage and onion in a large skillet over medium heat, stirring until sausage crumbles and is no longer pink; drain well. Return sausage to skillet; add 1 cup Cheddar cheese, cream cheese, and marjoram, stirring until cheeses melt.
Spoon 3 tablespoons filling down center of each Crêpe.
Roll up, and place, seam side down, in a lightly greased 13- x 9-inch baking dish.
Bake, covered, at 350° for 15 minutes. Stir together sour cream and butter; spoon over crêpes. Bake 5 more minutes. Sprinkle with remaining 1 cup Cheddar cheese and parsley. Serve with sliced tomato.
Note: Crêpes may be assembled and frozen. To reheat, let stand for 30 minutes at room temperature. Bake, covered, at 350º for 40 minutes. Proceed as directed.