Press ricotta cheese through a fine-mesh sieve into a large bowl. Add skim milk, mayonnaise, and sherry; stir well.
Drain spinach; press between paper towels to remove excess moisture. Add spinach and next 6 ingredients to cheese mixture; stir well. Spoon mixture into a 1 1/2-quart casserole coated with cooking spray. Cover and bake at 350° for 30 minutes. Uncover and bake an additional 10 minutes or until hot and bubbly. Serve with toasted pita wedges.