Preheat the oven to 425, as directed on the package of puff pastry
Lay out the puff pastry, and cut 6 equal rectangles. Lightly grease a muffin tin. Set both aside.
Cook the italian sausage until done. Set aside.
In the same pan, cook the leek slices with 1 tbsp butter until transluscent.
While the sausage and leeks are cooking, cut the potatoes into 1/4 inch cubes.
Line 6 muffin cups with the puff pastry. Press it gently down into the cup.
Layer in each cup: sausage, leeks, and potatoes. You should be able to do 2 thin layers in each cup. They will be heaping full.
Divide the remaining butter into thirds and put a piece on top of each muffin cup.
Fold the puff pastry over the top. You can gently pull and press the pastry so that it will completely cover the sides. Do leave a hole in the top for steam to escape!
Brush each mini galette with milk. Bake at 425 for 15-20 minutes, until the pastry is golden brown.