Melt butter in a pan over low heat. Add leeks, carrot, and celery; cook until softened, 5 to 7 minutes. Add fish parts, wine, and water and bring to a boil, about 5 minutes. Skim and discard residue. Add bouquet garni, peppercorns, and lemon; return to a boil. Reduce heat and simmer, uncovered, for 30 minutes; skim and discard residue frequently.