Heat the oil in a deep skillet or Dutch oven over medium-high heat.
Fry the plantain pieces until they begin to brown, about 2 to 3 minutes, turning a couple of times.
Remove the plantains and place them on paper towels. Let them cool completely. Reserve the oil.
Place each cooled plantain piece in a citrus press and press gently to form a cup shape.
Reheat the oil over medium-high heat.
Fry the cups until evenly golden, about 2 to 3 minutes, turning a couple of times.
Remove and drain on paper towels.
Make the dressing: whisk together the lime juice, soybean oil, agave, and salt until emulsified.
In a bowl, combine the mango, red onion, red bell pepper, and cilantro. Add the dressing and toss to combine. Refrigerate for 15 to 20 minutes.
At serving time, fill each plantain cup with 1 tablespoon of mango ceviche and garnish with a fresh cilantro leaf. Serve immediately.