Preheat the oven to 375 degrees F. Line 2 sheet trays with parchment paper or foil.
For the salad: Dice the sweet potatoes, beets and pears into similar size small cubes. Spread the sweet potatoes and pears evenly over one sheet tray and place the beets on the other sheet tray. Drizzle both sheet trays with olive oil and evenly sprinkle the sage and tarragon. Toss to coat and season generously with salt and pepper. Roast until vegetables are soft, 30 to 40 minutes.
For the sherry vinaigrette: In a small mixing bowl, combine the sherry vinegar, shallots and mustard, and stir to combine. Slowly whisk in the olive oil until emulsified. Season with salt and pepper.
Remove the vegetables from the oven and drizzle with the sherry vinaigrette just before serving.