Preheat oven to 350°F.
Wrap beets in foil and roast directly on oven rack for about 1 hour, or until tender. Remove from foil then care – fully remove the skins. Cool to room temperature, then cut into rounds.
In medium bowl, whisk together olive oil and lime juice until emulsified. Add garlic and let stand for 5 minutes. Remove garlic, add mangoes and toss to coat.
On plate or serving platter, arrange beet and mango slices in alternating layers, with each one peeking out from the other to showcase contrasting colors. Top with fennel fronds and drizzle with remaining lime and olive oil mixture. Serve immediately.
Serves 4.
