In a large roasting pan, big enough to hold all the bones, spread them out in one layer and coat them with a bit of vegetable oil. Roast the bones for one hour, turning them once or twice so they don’t burn.If you think they are getting too brown too quickly, turn down the heat some and don’t roast as long. When done, remove from oven and reserve.
About 15 minutes before the bones are done roasting, heat a large stock pot over medium heat, add two tablespoons of oil, and when hot, add the onions. Cook for 3 - 5 minutes.
Add the celery and carrots and cook for an additional 5 - 8 minutes. Transfer the bones to the stock pot and cover with water, about 12 cups.
Add the parsley and peppercorns to the pot. Bring to a boil and remove any foam or scum that forms.
Reduce the heat and cook at a simmer for about 2 - 2 ½ hours, skimming off any foam that comes back to the surface.
Take the stockpot off the heat and let it cool down before you strain it through a fine strainer. If you need to, use a colander and some cheesecloth. Discard the bones and vegetables.
Let the lamb stock cool down to room temperature before covering and refrigerating. You don’t want to work your fridge too hard by adding a hot container of stock to it.
As the lamb stock cools overnight, fat will surface to the top and get hard. Use a spoon to remove the surface fat and discard. The stock is now ready to use.
This homemade stock should be kept refrigerated and used within 4 days or frozen up to 4 months.