This past weekend, I made a mac-and-cheese beaucaire and substituted leek for eggplant. I oiled a cookie sheet, spread the leek on it, straight from the freezer, sprinkled with salt, and baked at 450°. After about 8-10 minutes, the leek has softened completely. They render a bit of fluid but not enough even to cover the bottom of the tray. Most of the leek browned nicely; some of the thicker stalks did not, but that can probably be managed by removing the browned ones first and let the rest roast for longer, or reduce the roasting temperature and let it roast for longer.
I imagine I could probably do something similar if I was to use the leek in a sautée I can also just take the leek out of the freezer and throw it straight into the sautée pan.