Preheat your oven to 450°F. If you have a crockpot with a removable insert you can usually put those into the oven. But check your manufacturer's instructions first. If not, you can use a dutch oven.
Put 2 cups of water in the pot and add all the beef bones and any offal you are using, veggies, thyme, parsley, bay leaves, and peppercorns. Roast in the oven for 30 minutes. After about 15 minutes, turn your crockpot on high.
Remove from the oven and transfer to preheated crockpot. Add the apple cider vinegar and cover with water. Leave the crockpot on high, cover the crock, and let it come to a boil. Skim off any scum that rises to the surface.
After it reaches a boil, turn to low and allow to simmer for a minimum of 12 hours, and up to 24. You will need to check it occasionally and add water as it evaporates.
Add the parsley and allow to simmer for 30 more minutes.
After it has finished cooking, you need to strain out all the solids. To strain, use a colander over a large pot with butter muslin inside it. Tie the corners together and hang for at least an hour.
After straining cover the pot or bowl and put it into the fridge for 8 hours to let the fat congeal on the surface.
Remove fat before using. You can store this in the fridge for about 2 weeks. For longer storage, the broth can be frozen, canned, or dehydrated into homemade bouillon.