Heat the olive oil and butter in a large saucepan. Once the butter has melted, add the onion and bay leaves. Sauté for about 5 minutes until the onion has softened, but not coloured.
Meanwhile cut the potatoes into rough 2cm pieces then add to the pan along with the garlic. Stir to coat in the butter then cover with a lid. Cook over a low-medium heat for about 8 minutes stirring once or twice to prevent the potato sticking to the base of the pan.
Stir in the mustard and pour in the stock, then bring to a simmer and cook with the lid off for 12 minutes until the potatoes are completely cooked through.
Remove from the heat and take out the bay leaves. Cool slightly before adding the milk and then blend well in a food processor or liquidiser until completely smooth.
Season to taste and serve hot, garnished with chopped chives and a twist of black pepper.