Preheat the oven to 200°C (400°F). Place the bones in a roasting tin and roast, turning once, until dark brown. Transfer the bones to a stockpot, cover with water and add the mushrooms. Cover the pan and cook gently for 3 hours. Add the toasted grains and continue to cook gently for 1 hour.
Remove from the heat and strain the broth. Simmer over low heat until reduced by half.