Follow the steps in order for the best result.
Roughly chop the onion, carrots, and celery into evenly sized pieces. Precision isn't crucial here.
Place the chopped vegetables and fish trimmings into a large pan. Add 1.2 liters of water, ensuring the ingredients are submerged. Introduce the parsley, thyme, bay leaves, crushed peppercorns, and garlic to the pan.
Bring the pan to a gentle boil over low heat. Once boiling, reduce the heat to a simmer and cook for 30 minutes. Skim off any impurities that rise to the surface during cooking. Avoid simmering for more than 30 minutes, as the stock may develop a slightly bitter taste.
Strain the stock through a fine-mesh sieve, ensuring no bones or small particles make their way into the final product. For extra clarity, strain it a few times.
Return the strained stock to a clean pan and boil for 10-15 minutes. This reduction step intensifies the stock's flavor.
Allow the stock to cool completely. Portion into smaller containers and freeze for later use.