I really wanted to eat a tsukudani-style simmer, so I bought a bunch of green celery. Celery leaves are actually kind of tough and hard, so instead of eating them raw it's common to have them softened in soups or tsukudani-like simmers. Those who dislike celery have even told me that they can eat it in this dish.
Don't flavor it from the start. Add after the leaves have softened a bit. You can simmer it until the juices have cooked off for a tsukudani, or enjoy as a simmer, both are delicious. Please adjust as desired.