Drain chopped spinach well. Gently press spinach between layers of paper towels. Set spinach aside.
Drain artichoke hearts, discarding liquid, 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 diced tomato and green chiles with liquid, 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 350º for 30 minutes.