Cook the whole, unpeeled potatoes in a pot of unsalted water to cover at a steady boil for 20 minutes or longer until just cooked through. Don't let them crack or get mushy on the outside. Let them cool briefly, then peel
and slice them into rounds, 1/4-inch thick.
Heat the oil and the bacon pieces in a skillet over medium heat, stirring occasionally, for 3 or 4 minutes until the bacon is sizzling and has rendered much of its fat. Stir in the leeks and cook another 3 minutes or so until they're wilted and sizzling. Spread the potato slices in the pan, season with salt and pepper and toss with the bacon and leeks
Cook for 8 minutes or more over medium-high heat, breaking up the potatoes into chunks. Spread and press the filling flat in the pan and let caramelize
for a couple of minutes. Turn the pieces, press and fry
, then repeat. When the filling is thoroughly cooked and has started to color, scrape it into a mixing bowl. Cool briefly then crush
the potatoes some more with a fork (I like to have little chunks left); add 1/2 cup Parmesan and salt and pepper, to taste.