ROAST, optional: If you’re using larger beef, lamb or pork soup bones that haven’t already been cooked, you can add extra depth of flavor by roasting them first. Preheat the oven to 400°F (205°C). Lay the bones out on a lined baking tray (or two, depending on your bones) and roast for 1 hour. Let cool slightly.
PREP: Place the bones into the slow cooker until it is about 2/3 full. Chop the carrots and celery into about 3 pieces each, then peel and quarter the onion. Nestle the vegetable pieces in and around the bones, then pour over the apple cider vinegar and add the herbs. Add salt, to taste, and then pour over enough water to cover the bones. Make sure to leave about an inch clear at the top between the water level and the lid.
COOK: Pop the lid on top and make sure it’s on securely. Set the slow cooker to LOW and cook for 12 – 24 hours (or even longer if you prefer). If your slow cooker has a vent or hole to allow for a meat probe, make sure to keep an eye on the water level in the slow cooker after the first few hours. If needed, add a little extra water throughout cooking to top off the water level and keep the bones covered. (I’ve never found this necessary myself).
STRAIN: Using oven gloves, carefully transfer the slow cooker insert to a trivet. Place a colander inside a large mixing bowl. Use tongs or a slotted spoon to remove any large pieces of bone and vegetables to the colander – this helps avoid messy splashing in a minute! Pour the rest of the broth and bits carefully through the colander and into the mixing bowl, setting the cooking container onto the trivet again once it’s empty. Once the liquid has drained through the colander completely, place it inside the empty Instant Pot insert to cool.
REPEAT: Place a fine mesh strainer on top of the gallon pitcher, then line that with a few layers of cheesecloth. Carefully strain the broth for a second time, pouring it from the bowl into the pitcher, through the strainer and cheesecloth to catch any remaining sediment or bone bits.
STORE: Carefully divide the broth, pouring it into wide mouth Mason jars. Pop the lids on and transfer them to the fridge. Once they have cooled completely, you can transfer them to the freezer if you like, but make sure to leave plenty of head room for the broth to expand if you are going to freeze it in glass jars! If your broth has a good layer of fat on top, it will keep in the fridge for at least a week and often much longer. Frozen broth can be kept for at least 6 months.
DISCARD: If you would like to save your bones to reuse, let them cool enough to handle first. Discard any herb stems, meat or pieces of fat. Go through the cooled bones, discarding any that are soft enough to crumble under a little pressure. Keep the remaining bones, transferring them to an airtight container and saving them in the freezer until you have enough bones to make a new batch of broth.