If it has been some time since the varathane was applied, then it should be strong enough to be scrubbed. Rub a mixture of baking soda and water over the surface and be sure to get inside all the corners. If possible do it in a warm-to-hot room to help the baking soda soak up the odor. You do not have to heat up the bread box, but warm temperatures will reduce the odors considerably. After that I would keep an open box of baking soda in the bread box the same way many people leave one in their refrigerator.