I often find I have a lot of leeks in the veg box, and I do throw them into stews, but the kids find them a bit stringy to manage like that, so I like to indulge in the following sometimes as it uses up lots of leeks but cooks down to small portions.
Depending on the size of the leeks, trim the ends off 2-3 leeks, run a slit down the length and chop into discs about half a centimetre thick.
Chuck into a lidded saute pan with a good generous ounce of butter, a spritz of olive oil and a very light sprinkling of salt. You do need the salt even if you don't normally add salt or the leeks don't break down properly. Stir around until the leeks are evenly coated and the butter is melted and cover, turn low, and leave it alone for 20 mins. At least. In my case I went to feed the smallest person and put her to bed. Come back and stir it about a bit and if you want to, add a little more butter. I know, it's decadent, but it's not every day, and it's worth it!
Cover again and leave, again on as low as possible, but keeping an eye on it not to blow out if it's gas. Leave for maybe another 15 mins, or as long as it takes to griddle some gammon, or grill a pork chop, or fry some bacon, or whatever you're having with this.
I also quickly scrubbed and diced a few potatoes, no peeling required, and microwaved them for 5 mins, then tossed in olive oil and a little salt, spread on a baking tray and bunged in a hot (220 deg) oven for the 20-30 mins the leeks took.
The leeks won't hurt being left longer as long as they don't burn, so keep the flame low, and remember you want the skin of the potato cubes to be crispy in places too.
No gravy needed, the leeks are so soft, they are like a sauce.