In A Saucepan, Saute Onions In Butter. Stir In Flour Until Blended. Gradually Stir In Milk. Bring To A Boil Over Medium Heat; Cook And Stir For 2 Minutes Or Until Thickened. Remove From The Heat. Stir In The Tuna, 1/2 Cup Of Potato Chips And Pepper. Pour Into Two Greased 8-Oz. Baking Dishes. Sprinkle With Remaining Potato Chips. Bake, Uncovered, At 350 Degrees F For 20-25 Minutes Or Until Hot And Bubbly.