In a large mixing bowl, combine the beef bones, onion, and tomato paste and mix well to coat.

Place on a baking sheet or lasagna pan and roast them in the preheated oven for about 2 hours, turning and rotating them every 30 minutes until they are well caramelized.

Once the bones are nicely caramelized, remove them from the oven and transfer them to the stockpot.

Add the chopped leeks, celery, parsley stems, black peppercorns, and bay leaves to the stockpot. Fill the stockpot with enough cold water to cover all the ingredients.

Place the stockpot on the stove over medium-high heat. Bring the mixture to a boil, then reduce the heat to a VERY GENTLE simmer. Skim off any impurities that rise to the surface.

Simmer the stock uncovered for 14 to 18 hours, maintaining a gentle simmer throughout. Stir occasionally and check to make sure it's not sticking to the bottom.

Once the stock has reduced significantly and has a rich flavor, strain it through a colander first to eliminate the bigger pieces.

Strain again using a fine-mesh strainer or cheesecloth into a clean pot to remove all remaining solids

Allow the strained stock to cool to room temperature. Then refrigerate it overnight to solidify the fat on the surface.

The next day, remove the solidified fat from the surface of the stock.

Bring the stock to a boil again and continue to simmer, reducing it to about half of its volume. This may take several hours, so be patient.

Once you've reached the desired concentration, the demi-glace is ready. It should have a rich, silky texture.

If you want to freeze the demi-glace in small portions, pour it into ice cube trays and freeze until solid.

Once frozen, transfer the cubes to a resealable plastic bag for easy storage.

Alternatively, you can store it in larger containers in the refrigerator but it will only last 1 week maximum.

When using the demi-glace in your recipes, simply add the desired number of cubes to your sauce or dish, allowing them to melt and infuse your dish with rich flavor.

Enjoy your homemade Demi Glace! It's a labor of love but well worth the effort when you savor the depth of flavor it adds to your dishes.