(Optional for brown stock) Preheat oven to 400°F. Place bones and chopped vegetables on a baking sheet. Roast for 45 minutes until browned.
Place bones and vegetables in a large stockpot. Cover with cold water.
Bring to a very gentle simmer over medium heat. Do not boil.
Reduce heat to low. Skim off any foam/scum that rises to the surface in the first 20-30 minutes.
Add parsley, thyme, bay leaves, and peppercorns.
Partially cover and simmer gently for 4 to 6 hours, occasionally skimming fat.
Carefully strain the stock through a fine-mesh sieve into a large bowl. Discard solids.
Let cool, then refrigerate. Remove solidified fat from the top before using.