Cook the manicotti shells just to barely tender, drain and cool. Mix the mettwurst, egg and cheeses; then stuff the manicotti shells with this mixture. Place in 8 x 10 pan or larger to hold all of the shells.
In saucepan sauté onion and garlic in butter. Add the flour; then slowly add the half and half. When mixture is thoroughly heated, add the Parmesan cheese and basil. Pour this mixture over the stuffed manicotti shells. Top with mozzarella cheese. Bake 35 minutes at 350°.
Note: This can be made ahead and refrigerated or frozen. If refrigerated, bake for 1 hour. If frozen, bake for 2 hours.