How to Make It
Cook spaghetti according to package directions; drain. Spoon into a lightly greased 13- x 9- x 2-inch baking dish; set aside.
Combine spinach and next 6 ingredients; spoon over spaghetti.
Remove casing from sausage. Cook sausage and onion in a large skillet over medium-high heat until meat is browned, stirring to crumble. Drain well. Sprinkle sausage mixture over spinach mixture.
Combine egg and ricotta cheese; spread over sausage mixture. Cover and chill 8 hours.
To bake, remove casserole from refrigerator, and let stand at room temperature 30 minutes. Bake, covered, at 375° for 35 to 40 minutes or until thoroughly heated. Sprinkle top with tomato and parsley.
Unbaked casserole may be frozen. To bake, thaw in refrigerator 24 hours. Remove from refrigerator, and let sand at room temperature 30 minutes. Bake, covered, at 375° for 50 minutes or until thoroughly heated. Sprinkle with tomato and parsley.