1) Place the coriander, spring onions, chillies, onions, lime juice and 200ml water in a small glass bowl or jar with a tight-fitting lid. Add the tomato, if using, season with salt to taste and stir all of the ingredients together. Cover and set the aji aside at room temperature for several hours to allow the flavours to release into the liquid, then refrigerate until serving.
2) Heat the oil in a large stockpot over medium heat for 1 minute. Add the onions and garlic and cook until they're soft and transparent, about 5 minutes, stirring occasionally. Add the tomatoes, bay leaves and thyme, and continue to cook 5 more minutes.
3) Add the chicken and the beef ribs and cook until the tomatoes have disintegrated, about 15 minutes, stirring occasionally and skimming the foam from the top of the stock when necessary. Add the green plantains, coriander and chicken stock, and bring to a boil. Reduce the heat to medium-low and simmer, covered, until the plantains are tender, 30 minutes.
4) Using a slotted spoon, remove the chicken from the pot and set it aside. Add the potatoes, pumpkin, ripe plantains, yucca and corn and simmer, uncovered, until both are tender, about 20 minutes. Remove the coriander leaves and the bay leaves and return the chicken to the pot to re-heat it.
To serve, arrange a piece of chicken, some beef, plantains and a few pieces of yucca and potato on each plate. Serve the stock in a small bowl and the aji sauce on the side, along with a bowl of rice, corn, some avocados and arepas or tortillas on separate plates.