In a large bowl whisk together the mustard, vinegar, olive oil, garlic, and salt.
Thinly slice the beets into 1/4 rounds. Thinly slice the shallot. Place them both in the bowl with dressing and gently fold together. You may eat them right away but they'll get tastier if you let them sit for at least an hour. Feel free to make ahead and store in the fridge overnight. Serve at room temperature.