can I freeze leftovers?
Wel, you certainly "can" in the sense that they would be safe for future consumption, but you would be highly disappointed in the bland, featureless mush that you would discover when you defrosted them!
I would purée with a little stock and then freeze . You can use purée in soups or sauces.
I just cook for the two of us, and don't really freeze much, so this is not from personal experience, but I read this at cooksillustrated.com: "When making a stew that you plan to freeze, leave out potatoes and squash. Cook them on the side and add them to the reheated stew before serving. More fibrous carrots, celery, celery root, parsnips, peas, and turnips were all fine." So from that I would surmise that all the veggies except potatoes and squash would be fine to freeze leftovers of. I would have thought so anyways, actually :) Plus the best way to know is just to try it, and then see if the texture bothers you. We all have such different taste in terms of what is acceptable to us.
I freeze roasted vegetables all the time. Yes, there is some degradation of texture after defrosting, but it most definitely is not a "bland, featureless mush." For sure, the freezing will not suck away all of the flavor.
Of course, some things hold up better to freezing than others, it's really up to you to experiment with this and decide for yourself what you deem acceptable.
Note that you have the option of repurposing the defrosted roasted veggies into new preparations. There is nothing that says you must eat the roasted veggies as is.
If you roast summer squash and freeze it, yes, the texture will definitely suffer, but if you toss said defrosted squash into ratatouille, a soup or a stew, well that could work out for you.