Ive been pretty much vegetarian before & after being diagnosed w/ UC. I say pretty much because when I'm flaring I eat organic burgers or steak to make up for blood loss.
When not in a flare, I find raw salads, lots of red onions and most veggies just fine. When in a flare as others have said, it's best to go w/ cooked vegetables and low fiber.
My feeling is, eat the rough stuff when you can because a flare with prohibit them. Also, Ive found kefir to be a great tool, red onions and other foods act as a prebotic to your probiotic kefir.
Best of luck on your journey, I believe being too restrictive w/ your diet can be a mistake that keeps you from adjusting to what your immune system is throwing at you at any given time.
so maybe eating the stuff that gets you out of a flare might also keep you from flaring again if you keep eating it ?