1 lb beets (4 to 6; preferably Chioggia*), 1 inch of stems left intact
3 tablespoons thinly sliced scallions
2 to 2 1/2 tablespoons raspberry vinegar
2 teaspoons fresh lemon juice, or to taste
2 tablespoons chopped fresh mint
1 tablespoon finely grated fresh orange zest (from 2 oranges)
1/2 teaspoon salt
1/2 teaspoon black pepper
1/4 cup olive oil
Garnish: fresh mint sprigs
Cover beets with water by 1 inch in a 2- to 3-quart saucepan and simmer until tender when pierced in center with a fork, about 30 minutes. Drain in a colander and rinse under cold running water. Let stand until cool enough to handle, then slip off and discard skins. Cut beets into 1/4-inch-thick slices.
While beets are cooking, stir together scallions, 2 tablespoons vinegar, lemon juice to taste, mint, zest, salt, and pepper in a bowl. Add oil in a slow stream, whisking until combined. Add warm beets and toss with vinaigrette and vinegar and salt to taste. Serve warm or slightly chilled.