Smoked mackerel, cauliflower and potato gratin

Method

1. Heat the oven to 180°C/fan160°C/gas 4. Bring a saucepan of salted water to the boil and add the potatoes. Put a steamer over the top with the cauliflower florets, cover and cook both for 5-7 minutes until just tender. Drain and leave to cool.
2. Meanwhile, make a béchamel sauce: melt 2 tbsp olive oil in a pan over a low heat, then stir in the plain flour. Fry for 2 minutes, stirring, then add the milk, bit by bit, stirring until thick and smooth. Grate in half the cheddar. Stir the mustard and spring onions into the béchamel. Season generously.
3. Layer the potatoes, cauliflower and mackerel fillets in a 2 litre ovenproof dish, then stir in the béchamel. Top with the reserved cheddar and a large handful of breadcrumbs, then bake for 20 minutes or until golden and bubbling.