Turn your oven to 375.
Scrub your potatoes and bring a pot of salted water to a boil. If you're using fingerlings, chop the largest in half but keep the small ones as is. For a larger potato, chop into quarters. Boil the potatoes till almost soft, 15 or so minutes depending on the size. Drain, and then cool. Slice the potatoes into small coins.
Heat up your butter over medium heat. Once melted, add in leeks and cook to soften, about 5-7 minutes. Add in about a tsp of salt.
Mix the potatoes, the leeks, a half tsp of black pepper and the rosemary together in a bowl.
Grease the bottom of a large casserole or pie dish. Spread the potatoes and leeks over the bottom.
Tear the turkey lunch meat up and spread all over the potatoes (you might not need all of it), removing the stringy bits as you go.
Carefully trim away the white rind from the cheese and drop chunks of cheese on top of the turkey.
Mix together the eggs and the milk or cream. Pour over top of the prepared fillings. The eggs should just about meet the top of the fillings, but it's okay if they don't (they'll grow in volume when baking.)
Bake covered in tin foil for 35 minutes, remove foil and bake an additional 25-30 minutes depending on your oven and the depth of your baking dish. The sides will start to pull away from the dish and the center should be firm, not jiggling, by the time it's done.
Allow to cool for 10 minutes before serving.