Place whole potatoes in a large saucepan and cover with water. Heat on high until water begins to boil. Cover and reduce heat to medium-low. Cook for 30 minutes or until potato is easily pierced with a fork.
Submerge potato in cold water to cool. While potato cools, dice green pepper and set aside, grate cheese and set aside.
Preheat oven to 400 degrees. Spray a casserole dish with non-stick cooking spray.
Drain tuna. Cut potato into slices. Layer half the potato slices in the casserole dish. Top with half the tuna and half the green peppers. Repeat.
Beat egg, milk and nutmeg together. Pour over casserole, top with grated cheese and place in hot oven. Bake for 30 minutes.