Serve this festive side dish with Pork Roast with Apples and Gremolata as part of your holiday dinner.
In a large pot of boiling salted water, cook green beans and parsnips until crisp-tender, 8 minutes. Drain; transfer to a large bowl.
In a medium skillet, heat olive oil over medium-high. Add red onion and cook until translucent, 3 minutes. Slowly add vinegar and mustard, season with salt and pepper, and stir to combine. Pour pickled onions and dressing over vegetables and toss well. Serve warm or at room temperature.