Boiled rice and a green salad are good on the side, though this is also the kind of dish people like to serve with hot buttered toast.
Heat the butter and groundnut oil in a saucepan and sauté the onion and pepper over a medium heat until soft. Add the garlic and chilli and cook for another minute, then add the flour.
Stir the flour in so it is well incorporated then take the pan off the heat. Gradually add the milk, just a little at a time, stirring well to mix the milk into the flour and vegetables. You need to add the milk gradually to ensure a lump-free mixture.
When all the milk has been added put the saucepan back on the heat and bring to the boil stirring constantly. Lower the heat and cook for another couple of minutes then add the cream, seasoning, mustard, Worcester sauce and lemon juice.
Heat the olive oil in a small frying pan and sauté the mushrooms over a fairly high heat for about 3 minutes. Season. You need to cook the mushrooms enough so that they throw out all the moisture in them – this has to be evaporated or it will make your dish watery once the gratin is in the oven.
Add the cooked mushrooms and the crab to the pepper and onion sauce and heat through. Taste for seasoning – it should be quite spicy.
Spoon the mixture into two lightly buttered gratin dishes. Toss the breadcrumbs with the parsley and grated cheese and sprinkle this on top. Drizzle the butter over them too.
Put into an oven preheated to 200 C and cook for 15 minutes, or until the top is golden and bubbling. Serve immediately.