1 pound baby yellow or red beets, trimmed, scrubbed
1/2 cup Sherry or red wine vinegar, divided
2 tablespoons plus 1/4 cup olive oil
1 medium shallot, finely chopped
1 teaspoon chopped fresh thyme
1/4 cup raw pistachios, chopped
2 tablespoons coarsely chopped fresh tarragon
Preheat oven to 425°. Combine beets, 1/4 cup vinegar, 2 tablespoons oil, and 1/4 cup water in a baking dish; season with salt. Cover with foil and steam until beets are tender, 40.50 minutes. Let cool slightly. Rub off skins with paper towels; cut beets into quarters (or 1/2" wedges if larger).
Toss beets with shallot, thyme, remaining 1/4 cup vinegar, and remaining 1/4 cup oil in a large bowl; season with salt. Let sit at least 2 hours.
Toss with pistachios and tarragon just before serving.
Do ahead: Beets can be marinated 2 days ahead. Cover and chill.