Preheat oven to 400 degrees F. Drizzle the marrow bones with a tablespoon of olive oil, shake pan, and then roast for 25 - 30 minutes or until bones are a rich brown hue. Remove from oven and set aside.

Leaving the skin on the yellow onions, trim the very tip of the roots then cut into fourths.

Set a large, 8-quart stockpot, over medium-high heat. Add two tablespoons of the fat from the bone roasting pan. Toss in the onions and sauté for 5 minutes. Next, add carrots, celery, garlic, bay, and peppercorns. Sauté for another 5 minutes.

Use tongs to add the roasted marrow bones (leaving any rendered fat behind) and pour in 5 quarts water. Bring mixture to a boil, and then turn heat down to medium-low, so mixture bubbles very gently. Simmer stock for 6 - 8 hours, adding 1 - 2 quarts more water as stock cooks down. I typically add just a pinch or two of sea salt – just enough so that I can taste the stock. Occasionally use a mesh spoon or strainer to skim any brown foamy stuff that rises to the top.

This method produces a very fatty stock. I like to cook my stock for 8 or more hours, until the marrow is very soft and any muscle or skin is falling off the bone. I then set pot in the fridge overnight (on top of pot holders) and skim and discard the thick layer of fat in the morning. Use tongs to remove large objects (like marrow bones), then strain stock and discard solids.

If using glass jars to store stock, ladle broth into a wide mouth quart jar, leaving 2-inches of head space. Set in the fridge overnight to chill. Then freeze, seal, label, and store for up to 4 months.