1. Preheat the oven to 425°F. 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 30 min to 1 hour, or until tender. Remove from the oven and cool for about 15 minutes.
2. 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.