Umi in Kamakura is a traditional Japanese-style restaurant where you can get super fresh fish and sashimi (it's called umi, the sea, after all) but they also have one vegetarian dish which is an incredible assortment of vegetables. Make sure to order the 季節の野菜ランチ (kisetsu no yasai ranchi - the seasonal vegetable plate).
This restaurant is a bit more expensive than others in the area but a great place to find local fresh ingredients.