Heat butter in saucepan. Saute sliced onions and garlic about 5 minutes. Then blend in the flour and milk with a whisk, add salt, pepper and stir over medium heat until thickened. Stir in drained tuna. Put quartered potatoes in greased 2 quart casserole dish and stir in tuna mixture. Cover and bake at 350 degrees for 1 hour. Uncover 30 minutes or until potatoes are tender.