To prepare plantains: Cut off ends of plantains and discard; slice the plantains in half crosswise. Place in a large pot and add enough cold water to cover by at least 2 inches. Bring to a boil over medium-high heat. Reduce heat slightly and simmer until the plantains can be easily pierced with a fork, 35 to 45 minutes. (Less-ripe plantains may need 5 to 10 minutes additional cooking time.) Add boiling water, if needed, to keep the plantains covered as they cook. Drain in a colander and let stand until cool enough to handle, about 5 minutes.

Peel the plantains, slice into small pieces, and transfer to a large bowl. Mash with a potato masher or fork, adding up to 1 cup hot water, a little at a time, to achieve desired consistency (like your favorite mashed potatoes). Stir in oil, salt, and pepper; cover and keep warm.

Meanwhile, to prepare picadillo: Heat oil in a medium nonstick skillet over medium-high heat. Add bell pepper, shallot, and garlic; cook, stirring often, until softened, about 2 minutes.

Add beef; cook, crumbling with a wooden spoon, until no longer pink, about 3 minutes. Add tomatoes, olives, capers, cumin, salt, and pepper. Cover, reduce heat to low, and simmer for 12 minutes. Keep warm.

To serve, divide the plantain mash among 4 bowls and top with the beef mixture. Serve immediately.