For the soup
Sauté the leeks in the oil for 15 minutes until soft (reserve a few finely sliced for garnish). Add the leftover potatoes and stock and allow to simmer for 15 minutes. Blend until smooth, serve garnished.
For the soda bread
Preheat the oven to Gas 4, 180°C, 350°F.
Place the flour, baking soda and cream of tartar in a bowl, cut the butter into small pieces and rub in with your fingertips. Make a well, pour in milk and stir with a knife blade until mixture forms a dough ball. Knead on a flour dusted surface for 4 minutes. Shape into a round, slightly flatten, then score surface with a cross.
Bake on a sheet for 30 minutes. Check if the bread is cooked by tapping on the base - it should sound hollow. Remove and allow to cool or eat hot from the oven.