Place the bones on a baking sheet and roast in a 400F degree oven for 40 minutes.
Remove from oven and place in the slow cooker, along with all remaining ingredients.
Fill the slow cooker to the top (leave about an inch or two) with water, cover and cook on high for 12 hours. Reduce heat to low and cook another 12 hours.
After cooking is complete, strain the beef broth using a strainer (or sieve/cheesecloth).
Divide into individual 1 cup servings and freeze in a freezer safe container for up to 6 months. If you plan on using it immediately, it should stay fresh in the fridge for about 5 days.
Notes
Don't skip roasting the bones. It adds an incredible flavor and your broth just won't be as good if you skip this part.
This recipe doesn't call for salt as I prefer to control the salt content depending on what I use the broth for.
Straining it through a sieve or cheesecloth will provide you with the clearest results.