Heat the oil in a large stockpot over medium-high heat. Add 10 tablespoons of the butter and let it melt. Add the meat scraps and stir with a wooden spoon to coat all the pieces with the oil and butter. Then add the carrots, parsley, celery, garlic, salt, and pepper. Cook, stirring frequently, until the meat is well browned all over. Remove the meat and vegetables from the pot with a slotted spoon and set aside.
Add 1 cup of water to the pot and scrape up all the browned bits from the bottom of the pot. Boil until almost all the water evaporates.
Return the meat and the vegetables to the stockpot and add enough water to cover by at least 2 inches. Bring to a boil over high heat, reduce the heat to medium, and simmer for 3 hours, stirring gently from time to time.
Add the remaining 6 tablespoons butter to the stock and stir until melted. With a slotted spoon, remove the meat and vegetables and discard. Strain the stock through a strainer lined with cheesecloth into a large clean pot.
Bring the stock to a boil, reduce the heat to medium-high so that it boils gently, and cook until it reduces by half to about 5 cups, about 20 minutes.
Pour into a bowl or other storage container and let cool. Store in the refrigerator for up to 3 days, or transfer to several small containers and freeze.