- Sea salt
- 500g potatoes
- 200g cavolo nero
- 2 onions
- 2 garlic cloves
- A handful of thyme, leaves only
- 200g cooked ham
- 2 tbsp butter
- Freshly ground pepper
- 2 tbsp plain flour
- 500ml full cream milk
- 200g mature Cheddar
- 1 tbsp wholegrain mustard
Prep: 20 mins | Cook: 30 mins
1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill a large pan with water. Sprinkle a little salt into the pan and bring to the boil. Scrub the potatoes, then chop them into 1cm-thick slices. Strip the cavolo nero leaves from their tough stalks and roughly shred the leaves. When the water in the pan is boiling, add the sliced potatoes and greens. Simmer for 5 mins, then drain through a colander and set aside to steam dry.
2. Meanwhile, peel the onions and thinly slice them. Peel the garlic cloves and finely slice them. Pick the thyme leaves off their sprigs. Roughly chop the ham into small chunks.
3. Melt 2 tbsp butter in a separate large pan on a medium heat. Slide in the onions, garlic and thyme leaves. Sprinkle with a pinch of salt and pepper. Fry for 6-8 mins, stirring occasionally, till the onions are slightly softened. Tip in 2 tbsp flour and fry for a further 2 mins, stirring.
4. Pour a splash of the milk into the pan with the onions. Stir well, till it combines with the flour into a thick paste. Slowly pour the remaining milk into the pan, stirring after every addition, till it's fully incorporated into a sauce. Bring the sauce to a simmer.
5. While the sauce simmers, coarsely grate the Cheddar. Stir the cheese into the sauce. Swirl in 1 tbsp wholegrain mustard. Add the chopped ham along with the drained cooked potatoes and greens. Fold till everything is coated.
6. Tip everything into a large, deep ovenproof dish and use a wooden spoon to level it out into an even layer. Slide the dish into the oven and bake for 20-25 mins till bubbling and golden. Scoop the hot gratin into bowls and serve.