To make filling, combine browned beef, browned sausage, sauteed onion and Marinara Sauce. Cook cannelloni as directed, but cook for only 3/4 recommended time; rinse and drain. Fill cannelloni with filling mixture and pack firmly into greased baking dish.
To make cheese sauce, warm White Sauce until nearly boiling; add grated cheddar cheese and stir until melted. Pour over cannelloni, coating pasta generously. Cool.
To freeze: cover baking dish with aluminum foil; place in freezer bag. Label and freeze.
To prepare: uncover and bake frozen casserole in 400-degree oven for 1 hour, or until hot and bubbling. Sprinkle with parmesan cheese, return to oven for 5 minutes to brown top.