To make the tostones: With a small sharp knife cut the ends from each plantain and cut a lengthwise slit through the skin along the inside curve. Beginning in the center of the slit, pry the skin from the plantain and cut the flesh crosswise into 1-inch-thick pieces. In a deep fryer or large deep skillet heat 1 1/2 inches of the oil to 375F. on a deep-fat thermometer, in it fry a batch of the plantain pieces for 1 1/2 to 2 minutes, or until they are pale golden, and transfer them with a slotted spoon to paper towels to drain briefly. Working quickly, using a tortilla press or the flat bottom of a glass or bottle, flatten each piece, a cut side up, between sheets of wax paper to a thickness of no less than 1/4 to 1/3 inch. Refry the flattened pieces in the 375F. oil, turning them occasionally, for 2 to 3 minutes, or until they are golden, transfer them with a slotted spoon to paper towels to drain, and season them with salt. (The tostones should be crisp on the outside and chewy on the inside.) Fry the remaining plantain pieces in batches in the same manner. The tostones are best served immediately, but they may be made 1 day in advance, wrapped well in plastic bags, and reheated on a rack in a shallow baking pan in a preheated 450F. oven for 3 to 5 minutes, or until they are heated through. Serve the tostones with the sauce. To make the sauce: In a heatproof bowl combine the onion, the scallions, the bell pepper, the garlic, the chili, the salt, and the thyme. In a small saucepan bring the water to a boil with the vinegar, pour the vinegar mixture over the vegetable mixture, and let the mixture cool. Stir in the lime juice and the oil. The sauce may be made 2 days in advance and kept covered and chilled. Let the sauce come to room temperature, stir in the chopped coriander, and garnish the sauce with the coriander sprigs. Makes about 2 cups. Makes about 24 tostones. Gourmet February 1993