Melt butter, then saute the garlic, onion, and leeks.Add in the potatoes, vegetable stock and let simmer with the lid on, until the potatoes are soft.Add in the cream, then stir in the Gruyère cheese until it starts to melt.If you like, use a stick blender to puree the leek and potato pieces, so it’s smooth and creamy (making it less stew, more soup).Spoon into bowls and top with a sprinkle of tarragon.