In A Skillet, Saute Onion In Butter Until Tender. Transfer To A Slow Cooker; Add Milk, Dressing, Fish, Broccoli, Cheeses And Garlic Powder. Cover And Cook On High For 1-1/2 To 2 Hours Or Until Soup Is Bubbly And Fish Flakes Easily With A Fork. Sprinkle With Paprika If Desired.