Put a large pan of water on to boil and place over it a steamer basket or colander. Thoroughly scrub, but don’t peel, 1.2kg of potatoes, cut into large pieces and put them in the steamer basket, covering them with a lid. Cook for about 25 minutes or until tender to the point of a skewer – it should slide in effortlessly. While the potatoes cook, trim 2 medium-sized leeks, discarding the roots and the very darkest of the green leaves (the paler green leaves are good here), then cut the leeks into rings about 1cm thick and wash them, very thoroughly, in cold water. Wash 200g of spinach leaves. Place a medium-sized saucepan over a moderate heat, add the still-wet spinach and cover tightly with a lid. Cook for 2 minutes, then lift the lid and turn the spinach over, cover and cook for a further minute or two until the leaves have relaxed. Remove the spinach and set aside. Return the pan to the heat, add 75g of butter and let it melt. Add the leeks, cover them with a lid and leave them to cook for about 20 minutes until soft. Keep the heat at a moderate level and stir them regularly, so they do not brown. When they are ready, briefly return the spinach to the pan. Lift the steamer basket or colander to one side, empty the water from the pan, then tip the potatoes into the empty pan. Using a fork or masher, crush them to a rough mash, then season. Tip in the cooked leeks and spinach together with any butter from the pan. Crush 6 juniper berries in a pestle and mortar or spice mill, fry them briefly in 30g of butter, season with black pepper, then pile the potatoes into bowls and pour over the juniper butter. You can prevent the leeks from browning by laying a piece of greaseproof paper over them, then covering with a lid to steam rather than fry. Leeks hold fine grit and soil between their layers. Washing them in a deep bowl of cold water is far better than in a colander under running water. This recipe is a base to which you can add other good things: crispy fried nuggets of pancetta or roughly torn rashers of grilled bacon; pieces of smoked mackerel or hot-smoked salmon. You could also use the mash as a stuffing.