Grind the peppercorns, clove and cinnamon together in a spice mill and mix with the sugar, sherry, garlic, vinegar and 1 1/2 teaspoons salt.
Spread 1/3 of the onion in the bottom of the crockpot; cover with 1/3 each of the tomatoes, chicken and fruit.
Add the bay leaves and sprinkle with some of the seasonings and pour on half the vinegar-spice mixture.
Repeat the layers, finishing up with a topping of onion, tomatoes and fruit; cover and cook for about 1 hour, then uncover and cook for 30 minutes longer, or until the chicken is tender and some of the juices have been reduced - Discard bay leaves.
FOR THE GARNISH: Heat the oil and fry the plantain slices until deep golden brown; remove and drain.
To serve, cover the top of the stew with the capers, olives and plantain.