Roast the bones on a baking sheet at 400 degrees until nicely caramelized, about 45 minutes. Place the roasted bones in a very large soup pot and add the onion, carrot, celery, peppercorns, bay leaves, and water. Cover, bring to a boil, reduce the heat, and simmer for 2-3 hours. Strain out and discard the solids, then place the stock into the refrigerator overnight.
In the morning, remove any fat that has solidified on the surface of the stock, then return the pot to the stove, uncovered, and bring to a boil. Let the stock boil until reduced to about 1½ cups. Pour the concentrated stock into ice cube trays and freeze. When solid, remove from the tray and store in a plastic bag in the freezer.
To use stock, dissolve 1 cube in 1 cup hot water and add salt to taste.
Recipe by Cooking With Michele® at https://cookingwithmichele.com/2015/06/a-better-roasted-beef-stock-think-less-storage-footprint/