Stir together lemon juice, cumin seeds, salt, and pepper in a medium bowl. Stir in oil and let stand while roasting beets. Put oven rack in middle position and preheat oven to 425°F. Tightly wrap beets in a double layer of foil and roast on a baking sheet until tender, 1 to 1 1/4 hours. Cool to warm in foil package, about 20 minutes. When beets are cool enough to handle, peel them, discarding stems and root ends, then cut into 1/2-inch-wide wedges. Toss warm beets with dressing. Stir in mint just before serving.