Peel and cut into large cubes, 3 to 4 pounds of potatoes (about 8). Spray bottom of roasting pan with Pam. Melt 1/4 cup butter. Pour 1 envelope Lipton onion soup mix in a plastic bag. Put raw potatoes into plastic bag and shake. Remove the potatoes from the plastic bag and place into the baking dish and spread evenly. Pour butter over potatoes. Bake at 350 degrees for 1 hour, turning occasionally.