I make an Au Gratin Leek Casserole. The first thing you need to do is clean your leeks really well, since they usually have some dirt, maybe even mud hiding at their base. I think the easiest way to do this is to chop them up and float them in a sink full of clear water - the dirt will fall to the bottom of the sink, while the leeks float on top of the water. Scoop them out with your hands, give them one more rinse for good measure, and dry them on paper towels.
Boil the leeks till they're tender. I make my own alfredo sauce with heavy cream, butter, and parmesan, but you could use bottled alfredo if you like. Mix the leeks with the sauce, butter a glass baking dish, and pour the mixture in. Sprinkle some parmesan on top, and bake at 350 for about 30 minutes. So yummy!