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.
This recipe is a keeper for family meals and I would make it for a potluck. My family and guests loved the flavor. I used it as a creamy sauce to mix into one 1 lb. of cooked mini Farfalle (bow tie pasta) and added cooked, cubed chicken breast. Also I provided Parmesan cheese to sprinkle on the dish. The pasta dish turned out to taste even better the next day as a left over.
For the casserole recipe as printed, I could not find canned artichoke hearts and so substituted 10 oz. of frozen artichoke hearts (defrosted and chopped into bite size pieces). To reduce the amount of spicy heat in the recipe, I used only half of the shredded Monterey Jack pepper cheese.