This recipe partially depends on the size of your crock-pot or other cooking vessel. Since we use a 6-quart crock-pot, the proportions given below are for roughly that size.

A few days before you plan on making the roast, take one larger chuck roast (3.5lbs or more) or two smaller roasts, at least one of which is a chuck or cut of roast that contains bone, and place them in a large glass container. Cover the roasts with buttermilk (½ cup water to ½ cup yogurt) and place in fridge for a day or two.

Come cooking time, remove roasts from fridge and rinse thoroughly.
(We sometimes cook our roasts over night, starting them a little after 9 or later for lunch the next morning, since it helps keep the house cooler by cooking at night and because I work afternoons)

Place the roast(s) in the crock-pot.

Place some carrots, celery, and onion in the crock-pot (Cost saving tip: You can save the tips of your carrots, celery, and onion pieces in the freezer that you would not use for eating or cooking but that are perfect for making stock in a recipe such as this one).

Add adequate water to fill the crock-pot, but not too full, as the liquid will usually expand as the roasts cook, so leave room for this expansion.

Add ½ cup or so of apple cider vinegar or lemon juice (something very acidic and alive but safe to help leach the bones).

Add a few smaller pieces of beef bone, especially marrowbones and rib bones, but any good-looking small bones will do well. They need to be small enough to be completely/almost completely submerged under the water. We usually add two beyond the bone in the chuck roast: a longer rib bone and then a heavier, rounder marrowbone.

Cook on high for the first hour or so, then reduce the heat (which for our crock-pot means turning it down to keep warm!) and allow the roast to cook for another 6 or more hours.

An hour to two hours or more before dinner, strain out the vegetables and, if you want, add some presoaked brown rice or potatoes to the crock-pot (make sure you turn the heat back up on the crock-pot).

If you do not add brown rice to the crock-pot (which absorbs a great deal of liquid, which when done with stock makes the rice far more nutrient dense), you should yield around or just under 3quarts of very good beef stock. It will not be as rich or as subtly flavorful as making it the longer way, where it simmers for 2-3 days, but it will be very nutritious and greatly improve the digestion, nutritional profile, and flavors of all foods involved, while also providing some extra stock for a few more meals to follow.