Question: I was attempting to roast a head of cauliflower with olive oil and seasonings so it comes out crispy. Following the instructions, I roasted it for 35 minutes at 450°F (the recipe said 30 minutes, but it didn't seem done), stirring once halfway through. However, after they cooled, it appears that they've steamed instead of roasted. My guess would be that I put too many on the sheet. Can I stick them back in the oven for another try? If so, for how long, and should I use more oil?

Answer: 
No, cooking them for too long will make them mushy, whether they’re roasty or not. You could try oiling then a bit and sticking them under the broiler for a couple of minutes (watch them closely!), but i wouldn’t cook them longer than that.
At that point, I'd try buttering and grilling them to add some brownness, and maybe a little cheese.

If the food has steamed, means there was a lot of moisture there in the first place. Could be you washed all the heads but they still had a lot of water, which steamed in the oven and kept the overall temperature too low to roast.

Your other option to save the food, is smash out a quick white sauce, dump in some grated cheese, and then pour it over the cauliflower for Cauliflower Cheese. Optionally grille that for a crispy brown top too.