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Heat oven to 180C/160C fan/gas 4. Spread the bones out on a baking sheet and roast them for an hour, turning them over after 30 mins.
Heat the slow cooker if necessary. Pack the veg into the slow cooker, add the bones and enough water to fill the pot to within 2cm of the top. Add the lemon juice and the bay leaves. Cover and cook on Low for 18-36 hours. The longer you cook the broth the darker it will become.
Place a colander over a bowl and scoop out all the bones into the colander. Return any broth from the bowl to the pan. Strain all the liquid through a fine sieve. Taste, and season only if you need to. Allow the broth to cool and lift off the fat. Store in the fridge for up to 3 days or transfer to freezer bags once it has cooled.