Preheat grill to medium–high heat. Cook for 4 minutes covered, flip, and continue to cook for 3 additional minutes covered or until internal temperature of 145°F is reached on instant read thermometer. Remove from heat, let rest for 3 minutes.
Make pickled onions with lime and salt.
Warm skillet over medium heat, add butter. Next place plantains in skillet making sure none overlap, cook plantains for 4 minutes until lightly brown.
Warm up black beans, and begin plate assemblance, Carne Asada, beans, plantains, and pickled onion on the side.