For the tangy pickled onions, combine the quartered red onion in a small bowl with a pinch of salt and a dash of water, letting it sit while you prepare the rest of the dish.
Peel the ripe plantain and cut it into chunks. Place in a small pot with a splash of water and a pinch of salt. Boil until very tender, about 10-12 minutes.
Meanwhile, pat dry the chicken breast and season both sides generously with salt, pepper, and finely chopped garlic and thyme leaves.
Heat the olive oil in a skillet over medium-high heat. Once shimmering, add the chicken breast and cook for about 4-5 minutes on each side, or until a golden-brown crust forms and the internal temperature reaches 165°F.
Once the plantains are tender, drain any excess water and mash them with a fork until creamy. Adjust seasoning with a pinch of salt if needed.
Remove the pan-seared chicken from the skillet and let it rest for a couple of minutes before slicing.
Plate the creamy mashed plantains as a base, arrange the sliced garlic-herb chicken on top, and spoon the tangy pickled onions over the dish.
Garnish with extra thyme if desired, and serve warm.