My partner is making breaded haddock. I have potatoes, beans, bok choy and arugula. What sides should I make ?

Or if the beans are green beans, you could make a potato and green bean salad, tossed in a pesto made from the arugula. I would try to sneak a little lemon juice in there somewhere too (maybe a squirt to brighten the pesto?... or maybe just wedges of lemon for the fish) though, for the acid.

Make colcannon: mashed potatoes but use the bok choy instead of cabbage.
https://food52.com/recipes/11988-how-we-survived-colcannon
I like sexyLAMBCHOPx's idea for arugula dressed with a light lemon vinaigrette.
Save the beans for another time.

Make your favorite GBD (golden, brown, delicious) potato dish for a fish n' chips feel; if those beans are green beans, steam them and toss with a little lemon juice or zest; turn that arugula into a salad.

Saute the potatoes, beans & bok choy and serve the arugula as a salad with a lemon vinaigrette.