Preheat the oven to 425. Toss the beets and squash with oil and sprinkle with salt. Arrange in a single layer on a rimmed baking sheet and roast for about half an hour, or until tender. Remove from the oven and cool for about 15 minutes.
Whisk the maple syrup, juice, vinegar, and mustard in a large bowl. Add the roasted vegetables and toss well. Add a few cracks of black pepper.
Cover and refrigerate at least 4 hours, then serve either chilled or at room temperature