Roast vegetables are good in themselves, and a surprising number of vegetables respond pleasingly to the treatment. Broccoli, cauliflower, parsnips, peppers, sweet potatoes, fennel, asparagus, carrots, green beans, onions, summer or winter squash, mushrooms -- any and all are delicious hot out of the oven, dressed only in olive oil, salt, and pepper. Roasted garlic scapes are outrageously good, and next spring you should be sure to make them in vast quantities. Roast eggplant is so delicious that I have pretty much given up on cooking it any other way; when I make dishes that call for fried or sauteed eggplant, I roast it instead (with plenty of oil - it's still usually less than it would soak up in a hot pan) and am never disappointed.
This last point raises the question: yes, what about roast vegetables as ingredients in more complicated dishes? When it is seven in the evening on a Wednesday, who wants to peel, cut, and roast several vegetables first, and then assemble them into something else? Not I. I do, however, very much like to reach into the refrigerator and mix and match from containers of several different vegetables that I thoughtfully roasted on the previous Sunday.
This gratin is good with nearly any combination of grains and vegetables that suit your fancy, though I would strongly urge you to include roast onions among them. What pleases your tastes and looks good to you at the market should be your guide otherwise. Add some greens, too, if you like, as I did in the batch shown in the photo. Cheese on top is good as well, though for some reason I seem to have skipped it in the photographed batch (foolish me), and it was still delicious.
Leftover grains work nicely here, and go with the theme of assembling pre-cooked elements into something else, though of course you can make something fresh for the occasion. I like using quinoa -- especially since I've started cooking it in the rice cooker -- or brown rice, or a mix of the two. Aside from the white sauce, which is the same one I use for baked pasta, quantities are very flexible.