Scrub beets; trim tops to 1 in. Place in a single layer in a large microwave-safe dish. Drizzle with water. Microwave, covered, on high until easily pierced with a fork, turning once, 14-15 minutes; let stand 5 minutes., When cool enough to handle, peel and cut beets into 3/4-in. pieces. In a bowl, whisk vinegar, oil, salt and pepper until blended. Add olives, beets and 1 tablespoon mint; toss to coat. Refrigerate, covered, until cold, at least 1 hour. Top with remaining 1 tablespoon mint.