Lentils cooked in tomato sauce with sautéed leeks. It's a one pot dish that I remember for at least 25 years but never have leeks enough to make. Simple seasoning, maybe some allspice and cloves.
I suspect they would also be excellent sliced in half length-wide, put slice side down on an olive oil greased baking sheet and roasted at high temperature (450 or so) until they start to caramelize. I can't think of a veg I've roasted using that technique that hasn't been good. It's easy to do a couple sheets on a weekend and snack on them out of the frig all week. Don't mix the veg up, keep each type on a separate part of the tray because they finish at different times. Green beans are faster than carrots. Each veg on the sheet in a single layer.
Leek and potato potage is a French classic. And
Cock-a-leekie pie (or
this one) from across the channel.
Braised
leeks and apples sounds good. I need to go inspect my leek patch, but I didn't have much success in growing them this year.