1. Put the oil, onion and three-quarters of a teaspoon of salt in a large frying pan for which you have a lid, then set it on a medium-high heat. Cook for eight minutes, stirring occasionally, until the onion is soft and lightly golden.
2. Add the vinegar, sugar and capers, and cook for another five minutes, until the liquid in the pan has reduced to about three tablespoonfuls.
3. Add the asparagus, artichokes, pine nuts and a good grind of pepper to the pan, and cook for two minutes.
4. Lay the fish fillets skin side up on top, cover the pan again and take off the heat. Set aside for five minutes, during which time the fish will steam and cook gently.
5. Meanwhile, put the peas and mint in a small bowl. Once the fish is just cooked, tilt the pan, transfer two tablespoons of the oil and a tablespoon of the vegetables to the pea bowl, and stir to combine.
6. To serve, carefully peel the skin off the fish – use your fingers or gently coax it away with a flat knife – and discard. Spoon the pea mixture on top and serve straight from the pan.