One of our most common vegetable side dishes around here is sauteed greens with onions.
I do occasionally use fresh collards, kale, mustard, or turnip greens for this dish, but most of the time I use frozen greens. It's quick, easy, very nutritious, and the children (while not crazy about it) can usually be coaxed to eat a bite or two. Fresh is a little better-textured, but not by a whole lot.
Normally I saute onions in plenty of olive oil with salt and pepper until they are soft and maybe starting to brown a bit; sometimes add garlic; and then dump the frozen greens into the pan. Stir from time to time, and when it's all hot, add more salt and pepper, plus a bit of lemon juice or vinegar, and call it good. It takes a total of 15 minutes, and of course you can let it go for longer, or if you want the greens to be really mushy you can add some liquid and cover the pan to steam them. I like them still chewy.
Couple of days ago I had some extra celery, so I coarsely chopped three stalks of celery along with my two medium onions for the collard greens. Did everything the same, only with the celery.
I couldn't believe how much better it was! The bitterness of the greens was tempered a bit, and the celery added a nice crunch. I almost always have celery in the fridge — and this sort of thing is still good even when the celery has started to go a little flaccid — so I think I'm doing the collards this way from now on.