Gently poaching fish in a flavorful liquid just about guarantees a perfect outcome—juicy, tasty fillets. Bonus: With this "Trinity" Poached White Fish recipe, you also get vegetables and a delicious sauce, all in one pan. The “trinity” in classical French cooking (onions, carrots, and celery) is the foundation for dozens of stocks, soups, stews, and sauces. Here I’ve used leeks in place of onions. Be sure to rinse them well; they grow in sandy soil, and often have a bit of grit between the layers.