Confit mackerel with rosemary, garlic, potatoes and peppers. Photograph: Harry Weir Photography

1. Set the oven to 150 degrees

2. Scatter the potatoes, peppers, garlic, orange and thyme in a large roasting tray, then add the two oils.

3. Season and cook for approximately 30 minutes until the potatoes are tender.

4. Remove from the oven then slip the mackerel on to the tray and baste with the oil.

5. Cook for 10 more minutes, remove from the oven and allow to cool.

6. Lift the mackerel, potatoes and peppers carefully from the tray and put on a large plate or dish to chill in the fridge.

7. Strain the oil through a sieve.

8. Put the egg yolks into a bowl with the lemon juice, harissa and salt.

9. Whisk the oil into the yolks in a steady drizzle as if making mayonnaise.

10. When it’s all incorporated it’s ready.

11. To serve, artfully arrange the mackerel, potatoes and peppers on plates with the dressing on the side so everyone can help themselves.