Start by blanching your beef marrow bones in a big pot of boiling water. Let them simmer for about 10 minutes.
Preheat oven to 400 degrees.
Transfer the blanched bones to a roasting pan and roast them for 30 minutes.
Transfer the roasted bones to a large stockpot.
Add the cubed onion, garlic cloves, carrots, and a splash of apple cider vinegar.
Add optional seasonings like sea salt, bay leaves, and oregano to your taste.
Fill the pot with cold water, enough to fully submerge all the ingredients.
Bring the mixture to a boil over medium-high heat, then reduce to a very low simmer.
Allow the broth to simmer on low heat for 8-12 hours.
After simmering, remove the pot from the stove top. Strain the broth using a fine-mesh sieve to remove the solids.
Allow the broth to cool to room temperature. You may notice a layer of fat on top of the broth; you can skim off this excess fat.
Store the strained broth in a large bowl in the refrigerator overnight. Any remaining fat will solidify at the top, making it easy to remove.
Pour the broth into ice cube trays or small containers for easy use later. You can freeze the broth for several months.