PreparationMake stock: Bring fish bones, shrimp shells, tomatoes, onion, bell peppers, cilantro sprigs, parsley sprigs, lime juice, salt, garlic, plantain, and water to a boil in a 5- to 6-quart pot, then reduce heat and simmer stock, uncovered, until liquid is reduced to about 4 cups (vegetables will exude liquid as they cook), about 25 minutes. Pour stock through a fine-mesh sieve into a large bowl, discarding solids. Measure stock: If there is more than 4 cups, boil until reduced; if there is less, add water. If using stock right away, bring to a boil in cleaned pot; otherwise, chill, uncovered, until completely cooled, then cover. Make soft manioc polenta: Add manioc flour in a thin stream to boiling stock, whisking constantly, then whisk in salt. Reduce heat to moderately low and cook, whisking, 1 minute. Stir in chopped herbs and let stand 2 minutes. Serve immediately. Cooks' note:Stock can be chilled in an airtight container up to 3 days or frozen 3 months. Bring to a boil before adding manioc flour.