What are some good side dishes to go with bison short ribs with mustard?

Love short ribs! How about a simple risotto, wild rice pilaf, or good ol' mashed potatoes to capture the juices? Lentils or pureed cauliflower could be nice too.

polenta is good, but so would just be some roasted root veggies like celery root, beets, parsnip, carrots, rudabaga, etc.

I also like polenta, but have also served roasted fingerling potatoes as well as roasted sweet potatoes. I like a big green salad to cut the richness of the short ribs and something lemon for dessert.

Old fashioned I know but I would totally go with some thick-cut steak fries (yum!) and sauteed mushrooms with spinach. Saute the mushrooms in butter/olive oil until done add salt and pepper to taste and then add 1/4 cup water, fresh spinach and cover, turning off the heat and letting it wilt just a bit. Finish with a squeeze of lime juice.

When I do buttered noodles with a short rib, I like to add a parsley-lemon gremolata on top to cut the richness. My favorite thick pasta is mama coppola's farfallone. My local store no longer carries it, so I order it by the case.

I love short ribs over polenta or wide pasta noodles. I typically just do a fresh salad to go with it as it's heavy enough as it is.

A creamy polenta would pare great with that as a starch element.
Slaw would be another veggie option--perhaps as the salad course, or a Arugula/spinach/spring greens salad.
kale with bacon would work also for a warm veggie dish.
Asparagus. would be great. I highly recommend this application.
http://www.food52.com/recipes/4023_absurdly_addictive_asparagus
You can leave out the pine buts and put in some walnut, or skip the nut option.
A warm cabbage salad---chopped cabbage, sauteed with onions and apples in bacon or duck fat, finished with caraway seed and vinegar.
Think Autumn and German for the sides. But don't overlook potlenta or risotto.