with dauphinoise you probably don't need much of a sauce and you probably want something green to cut through the richness. So something like chard would work. If you want to do something like a parsley sauce, i would cook baked potatoes (or boiled anya potatoes) instead and serve with red cabbage
I'm doing a giant ham with daupinoise tomorrow. Am adding spiced red cabbage (cooked long and low) and then for some crunch lightly cooked sugar snap peas and chunky carrots. SIL is bringing a cranberry sauce to go with it. I was hoping the lashings of cream in the spuds would keep it moist.