Pressure Cooker/Instapot: Add all ingredients to pot. Pressure cook on high for a total of 6 hours for beef. It is important to cook your stock for over 4 hours to allow maximum extraction of proteins and minerals (especially for dense beef bones.) It also caramalizes the vegetables and creates rich deep flavors. This also eliminates the need to pre-roasts the bones and veggies.
Stove-top: Combine all ingredients in a large stock pot. Bring to a boil and reduce heat to low. Cover and simmer for 48 hours. Add more water as it evaporates. Note: It is not recommend to do this unmonitored with a gas stove.
Slow-cooker: I find that most slow-cookers are lined with a non-stick lining. I highly recommend ditching any non-stick coated cookware. If you do want to use the slow-cook method, just combine all ingredients in your slow cooker and slow cook on high for 48 hours. Monitor the water level and add more water as it evaporates. **note: Some slow cookers don't get hot enough to make a really thick broth.
Now the fun part! Strain out the bones and ingredients. I place a colander in a large bowl and pick out the bones with tongs, a mesh basket or a small strainer. Once you get all the big pieces out it is a good idea to strain the broth through a fine mesh sieve to get any large chunks or pieces of bone out. At this point you can either use a gravy fat separator, or just place your broth in the fridge and wait for the fat to harden so you can remove it. The fat can be used to fry potatoes or any other foods.
Store bone broth in the fridge for 1 week or freeze in small containers or ice cube trays. Use within 2-3 months for best results. Spent bones are a great addition to a garden compost, especially if charred in a fire pit first.