Preheat oven to 350°.
. Cook pasta according to package directions, omitting salt and fat. Drain.
. While pasta cooks, melt butter in a Dutch oven over medium heat. Add onion and garlic; sauté 4 minutes or until tender. Lightly spoon flour into a dry measuring cup; level with a knife. Add flour to pan; cook 1 minute, stirring constantly. Add milk, stirring constantly with a whisk. Cook 11 minutes or until thick. Add spinach, stirring until spinach wilts. Remove from heat; add 1/4 cup cheese, salt, and pepper.
. Stir in chicken, salsa, and 3/4 cup cheese. Carefully fold in pasta. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray. Sprinkle evenly with remaining 1/4 cup cheese. Bake at 350° for 39 minutes or until bubbly.