1 package (10 ounces) frozen chopped spinach, thawed and drained
5 slices day-old bread, crumbled
1/2 to 2/3 cup beef broth
1/2 cup grated Parmesan cheese
1 egg, beaten
1 tablespoon minced fresh parsley
1/2 teaspoon salt
1/4 teaspoon pepper
1/8 teaspoon ground nutmeg
Peel onions and cut 1/2 in. off tops and bottoms. Place onions in a large saucepan. Cover with boiling water. Cook until tender, about 20 minutes; drain.
Cool slightly. Carefully remove inside layers of onion, separating into eight individual shells (refrigerate remaining onion for another use). Drain on paper towels.
In a skillet, cook beef and sausage over medium heat until no longer pink; drain. Add spinach; cook and stir for 2 minutes. Remove from the heat; stir in the remaining ingredients. Spoon into the onion shells.
Place in a greased 13-in. x 9-in. baking pan. Bake, uncovered, at 350° for 15-20 minutes or until heated through and lightly browned.