Preheat oven to 400°F (200°C). Arrange beef bones on a baking sheet and roast for 30 minutes until browned. This step adds depth of flavor.

While bones roast, char the halved onion and sliced ginger directly over an open flame (or under a broiler) until slightly blackened and fragrant. This enhances their aroma.

In a large stockpot, combine the roasted beef bones, charred onion, charred ginger, rock sugar, salt, fish sauce, and the spice bag. Add 1 gallon of water to cover all ingredients.

Bring the mixture to a rolling boil, then immediately reduce heat to low. Cover with a lid and let it simmer gently. Skim off any impurities that rise to the surface during the first hour.

Cook for a minimum of 2 hours as an initial phase. For a truly rich and gelatinous bone broth, continue simmering for a total of 6-8 hours (or even longer, up to 12 hours) on very low heat, maintaining a gentle simmer.

After the desired cooking time, carefully remove the beef bones and discard them. The snippet mentions removing bones after 2 hours; however, for maximum flavor and collagen, simmering them longer is ideal. If removed earlier, cover the pot again.

Strain the broth through a fine-mesh sieve, ideally lined with cheesecloth, to remove all solids. Discard the solids.

Taste the broth and adjust seasoning with additional salt or fish sauce if needed. Your rich Vietnamese Beef Bone Broth is now ready to be used as a base for pho or other dishes, or stored for later use.