Drain chopped spinach well, reserving 1/2 cup liquid. Gently press spinach between layers of paper towels. Set spinach aside.
Drain artichoke hearts, discarding liquid, and cut in half.
Melt butter in a 3-quart saucepan over medium-high heat; add onion, and sauté until tender. Add flour; cook, stirring constantly, 1 minute. Stir in reserved spinach liquid and milk; cook, stirring constantly, 2 minutes or until thickened and bubbly. Stir in spinach, artichoke, garlic salt, pepper, and Worcestershire sauce. Spoon into a lightly greased 1 1/2-quart baking dish. Sprinkle with cheese.
Bake casserole at 350° for 30 minutes.
Spicy Spinach-Artichoke Casserole: Substitute 1 (10-ounce) diced tomato and green chiles, undrained, for milk and spinach liquid and 3/4 cup shredded Monterey Jack cheese with peppers for Parmesan cheese.