Beef and Spinach

4 tablespoons butter
1 tablespoon chopped onion
1 pound ground beef
1 teaspoon salt
1/2 teaspoon sage
1 egg
1 cup milk
1 1/2 cups fine bread crumbs
2 1/2 cups cooked spinach

Melt butter in skillet, add onion and brown slightly. Add the beef, salt and sage and fry until brown, keeping the beef in separate particles. Beat the egg and add the milk. Butter a square or oblong glass baking dish. Place half the bread crumbs on the bottom, then half of the meat mixture, and next all of the well-drained spinach. Top this with the rest of the meat and the remaining bread crumbs last. Pour the egg and milk mixture over the top. Bake thirty minutes. Serve in squares. A pound can of salmon may be substituted for the beef it desired.