Peel the potatoes and cut them into chunks.
Add the potatoes to a large pot of cold salted water. Bring to the boil, and boil til tender. Drain and set aside.
While the potatoes are cooking, peel and trim the leek, removing the tough outside leaves.
Cut the leek in half lengthways, and then each half lengthways again. Dice the lengths of leeks cutting across the stalks.
Place the diced leeks into a fine mesh sieve and rinse well under cold running water and let drain.
Heat the oil and butter in a skillet on a medium high heat.
Once hot, add the leeks and cook them, stirring occasionally til they start to brown. Add in the minced garlic, cook for a few more minutes then take off of the heat and place to one side.
In the pot that you boiled the potatoes in, add the milk and a tablespoon of butter and heat til the butter has melted.
Place the potatoes into the milk mixture and mash.
Stir in the sautéed leeks.
Taste and season with salt and pepper to taste.