Heat the oven to 220C/200C fan/gas 8. Toss the sweet potatoes with the oil and some seasoning on a baking tray. Roast for 20 mins.

Combine the yogurt, mayonnaise, cornichons and reserved brine, the shallot and dill with 1 tbsp cold water in a small bowl and set aside.

Meanwhile, put the peas in a pan with the milk, bring to a simmer and cook for 5 mins. Blitz the mixture using a hand blender until roughly puréed. Stir in the mint and season to taste. Set aside.

Add the cod or pollock to the baking tray with the sweet potatoes, season and cook for 10-15 mins more, or until cooked through. Warm through the pea mixture. Scatter over some dill and serve the traybake with the yogurt tartare and the mushy peas.