Rinse and peel sweet potatoes and russet potatoes. Cut potatoes into 1-inch chunks. Put in a 2-inch-deep 12- by 17-inch pan. Add garlic and 1/4 cup melted butter.
Bake in a 475° oven for 45 minutes, turning vegetables with a wide spatula every 15 to 20 minutes or so. Then stir in hazelnuts.
Continue to bake potatoes until they are very tender when pierced and their edges are well browned, about 20 minutes (30 minutes if cool); occasionally turn pieces with wide spatula. Stir in remaining 2 tablespoons melted butter. Add salt to taste.