1. Boil the potatoes in a small amount of salted water.
2. While the potatoes are cooking, turn one can of the tuna, bit by bit, with its oil into the container of an electric blender. Add one tablespoon of the milk at a time and blend until smooth. Repeat the process until all the tuna and milk are used. If a blender is not available, mash the tuna very fine and mix with the milk.
3. Peel the potatoes, mash and whip until fluffy with the butter, mustard, salt, pepper and tuna puree. Add more milk if necessary to give a fairly firm mashed-potato consistency.
4. Turn the mixture into a well-oiled five- or six-cup ring or fish-shaped mold. Chill.