Heat the olive oil in a medium saucepan over medium-high heat for 1 minute. Add the onions, carrots, celery, and garlic and cook until the onions are soft and browned, 8 minutes, stirring often with a wooden spoon. Add the chicken stock, plantains, cilantro, bay leaves and cumin, and bring to a boil, reduce the heat to the lowest setting and cook at a bare simmer, uncovered, until the plantains are very tender, 45 minutes.
Transfer half of the soup to a blender and puree until smooth. *When blending hot liquids, fill the blender less than halfway full, place the lid askew and pulse the liquid at first to release some heat, otherwise, your blender top could pop! Stir the pureed soup back into the pot with the remaining chunky soup and mix well. If the soup is too thick for your taste add more chicken stock. Season with salt and pepper and serve.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.