Peel potatoes, rinse and cut into 1/8 inch thick slices (should be about 4 cups.
Place in a bowl with chopped onion and toss to distribute. Mix flour, salt, pepper, mustard, and turmeric and sprinkle it on top of the potato onion mixture Toss and stir to evenly coat.
Place in a greased casserole dish distributing evenly.
Cut the butter into pats and place on top of the potatoes. Pour milk over all.
Cook uncovered in 325° F. oven 1 hour and 45 minutes until potatoes are tender.
Let stand 5 to 10 minutes before serving.