The sweet earthiness of roasted beets combines with the tang of citrus and the peppery bite of watercress in this jewel-colored salad. For maximum flavor, serve the salad at room temperature or just slightly chilled.
Position a rack in the lower third of the oven and preheat to 400 degrees F.
Rub the beets with olive oil and place in a shallow roasting pan. Cover the pan with aluminum foil. Bake until the beets are tender when pierced with the tip of a knife, 40 to 60 minutes, depending on their size. Remove the pan from the oven and let the beets cool. When cool enough to handle, unwrap the beets and peel. Cut the beets into 1/2-inch-thick slices or wedges and set aside in a small bowl.
Meanwhile, grate the rind (zest) from 2 of the oranges. Cut one of these oranges in half crosswise and squeeze the juice into a small bowl. Add the zest, vinegar, and shallots, and let the mixture sit at room temperature for 15 minutes. Whisk in the Dijon mustard, olive oil, thyme, salt, and pepper. Alternately, you can combine all of the dressing ingredients in a small jar and shake vigorously to emulsify the vinaigrette.
Add 1 to 2 tablespoons of the vinaigrette to the beets, tossing to coat.
Slice off the tops and bottoms of the remaining 2 oranges. Then, with a sharp knife, cut off the peel and pith (white membrane) in vertical strips working from the top down, following the natural curve of the fruit. Release the segments by cutting on the inside of each membrane.
Place the watercress in a large bowl and drizzle with some of the vinaigrette, tossing to coat the leaves. Transfer the watercress to a platter and arrange the beets and orange segments atop the greens. Serve immediately.