Preheat the oven to 400˚. Generously brush a 24-cup mini-muffin pan (preferably nonstick) with olive oil. Bring a large pot of salted water to a boil. Add the plantains and cook until very tender, 12 to 15 minutes. Drain and let cool slightly.
Using your fingers, press a plantain chunk in the bottom and up the sides of each muffin cup to form a crust. Bake until browned around the edges, 25 to 30 minutes. Let cool 5 minutes, then remove from the pan.
Meanwhile, combine the avocado, tomato, bell pepper, red onion, serrano, lime juice, cilantro, olive oil and 1 teaspoon salt in a medium bowl. Toss gently, then divide among the plantain cups.