MAKE THE SOUP: Melt the butter in a large, heavy saucepan. Add the carrots and onion and season with salt and pepper. Cook over moderately low heat, stirring occasionally, until the vegetables are softened and golden, about 30 minutes. Add the scallions, Scotch bonnet, soy sauce, thyme, ginger, cumin, allspice and nutmeg and stir until fragrant, about 4 minutes. Add the stock, potato and bay leaf and bring to a boil. Cover and simmer over low heat until the potato is tender, about 25 minutes.
Remove the soup from the heat and let stand for 10 minutes, then remove the bay leaf. Puree the soup in batches in a blender and return it to the saucepan. Season the soup with the lemon juice and salt and pepper.
MAKE THE PEAR RELISH: In a medium bowl, toss the pears with the lemon juice. Fold in the parsley, allspice and nutmeg. Chill the relish.
MAKE THE PLANTAIN CHIPS: Peel the plantain and thinly slice it on the diagonal. In a large skillet, heat 1/4 inch of oil until shimmering. Add half of the plantain slices and fry over moderate heat until golden brown and crisp, about 2 minutes per side. Drain on paper towels while you fry the rest. Season with salt just before serving.
Reheat the soup. Ladle into shallow bowls and garnish with the cold pear relish. Serve with plantain chips.
Make Ahead: The soup can be refrigerated for up to 3 days and the pear relish for 1 day. The plantains can be fried early in the day; reheat them in the oven.