Preheat the oven to 400°F.
Roast the beets. Note: I like to do this step up to a few days in advance so that the beets can chill before layering into the salad. Place each beet on a piece of foil and drizzle generously with olive oil and pinches of sea salt and freshly ground black pepper. Wrap the beets in the foil and roast on a baking sheet for 35 to 60 minutes, or until soft and fork-tender. The time will depend on the size and freshness of the beets. Remove the beets from the oven, remove the foil, and set aside to cool. When they are cool to the touch, peel the skins. I like to hold them under running water and slide the skins off with my hands. Chill until ready to use, then slice into halves or quarters.
Roast the carrots. Raise the oven temperature to 450°F and line a baking sheet with parchment paper. Roast the carrots for 15 to 20 minutes, or until tender. Remove from the oven and slice them in half.
Assemble the salad with the watercress, roasted beets, and roasted carrots. Drizzle with half the dressing, and top with the avocado, pepitas, and microgreens. Drizzle with more dressing and serve extra dressing on the side. Season to taste with salt and pepper, if desired.