I've been giving him little bits of torn up kale when I give him his lunch ( grower feed), and he eats that. I hear ppl say they give their chickens kitchen scraps, like vegetable peels and rinds. I suppose those need to be cooked also.
Vegetables don't need to be cooked. With meat the problem with raw would be the risk of things like salmonella and e. coli, and maybe even parasites. Cooking kills that. With veggies there really isn't that concern. This time of year mine like it when I hang up a head of cabbage or lettuce for them to go after. Gives them something to do

Other things they like are pumpkins, cucumbers, tomatoes, and any kind of bread.