Make slits with the tip of a knife all over the cod. Slice the cloves of garlic and press them in slits. Season with salt and pepper, place on a baking tray or roasting pan and drizzle with olive oil. Roast in the oven for approximately 45 minutes. When it easily comes off the bones, it is done. Remove the cod from the oven and set aside while you prepare the chanterelles.
Heat a saute pan over high heat. Add the butter and the oil and heat. Add the shallots and saute for 1 minute. Add the chanterelles and cook until they are tender and have given up their liquid. Season with salt and pepper and serve with the cod.