Put onions, garlic and olive oil in bottom of slow cooker. Add tin chopped tomatoes and top up tin with water and add to slow cooker. Shake some parsley in over the top and add a generous slosh of white wine. Finally add the coley, sliced mushrooms and potatoes. Give all a stir to ensure coated.
Cook on high for 2 to 3 hours or until potatoes are soft. Serve alongside some rice or pasta or simply by itself in a warm bowl.