A lovely warming autumn dish. You'll need a casserole dish big enough that the leeks (chopped into a couple of pieces each) can all sit flat. Serves two as a main course (or 3--4 as a side).
Preheat the oven to 180 degrees. Melt the marge, half each in two separate pans. One of them will be for making the white sauce. Put the lardons into the other one, on a low heat, just so they warm up and fry a tiny bit and flavour the marge.
Wash the leeks and prepare them for your casserole dish. Chop them into two or three pieces, as needed, and tile them into the bottom of the casserole dish so they form a single layer.
In the other pan, on a medium heat, start to make the white sauce. Sprinkle about half of the flour into the pan, and whisk it until smooth. The lardons in the other pan should have had a few minutes to warm up - turn the heat off for them, and pour the juices from the pan into the one where you're making the sauce. The idea is to get some of the bacony flavour into the sauce.
Put the lardons to one side. Put the rest of the flour into the sauce pan, and whisk again until smooth. Now continue to cook this "roux" for a couple of minutes, so the flour is cooked, then gradually add the milk (with whisking) and continue to cook for another couple of minutes.
Now assemble. You've already got the leeks in the bottom of the casserole dish; sprinkle the lardons over them, then gently pour the sauce evenly over. Finally crumble the feta on top. Cook in the preheated oven for about 40--45 minutes, until nicely browned on top. Serve with salad.