Whisk together vinegar, mustard, salt, and pepper. Add oil in a slow stream, whisking until emulsified.
Roast beets and potatoes:
Put oven racks in upper and lower thirds of oven and preheat oven to 425°F.
Cut beets into 1/3-inch dice and put in a large salad bowl. Cut potatoes into 1/3-inch-thick slices and add to beets along with all greens and herbs. Add vinaigrette and toss gently to coat.
Sprinkle blossoms on top and serve immediately
*microgreens such as baby Bibb, red and oak-leaf lettuces, baby arugula or mesculin
**Available at farmers' markets and specialty produce markets