With the rack in the middle position, preheat the oven to 190 °C (375 °F). On a large baking sheet, toss the potatoes and sweet potato with the oil. Season with salt and pepper. Bake for about 1 hour, tossing them halfway through cooking, until the fries are tender and golden brown. Keep warm.
Fish and Chips
In a bowl, combine the flour, cornstarch, salt, baking powder, and turmeric. Graduating add the water, stirring with a whisk. Set aside.
Pat the fish fillets dry with paper towels. Dip the fillets in the batter and coat thoroughly.
In a large non-stick skillet, brown half the fillets at a time for about 2 minutes on each side in the oil. Serve with yogurt tartar sauce.