Heat your oven to 200°C/Fan 180°C/Gas 6. Put a large pan of salted water on to boil
Finely grate the zest from the lemon and orange into a bowl. Pick the parsley and thyme leaves off their stems, then finely chop (if the thyme stems are very soft you can chop them too). Add them to the citrus zest with a pinch of salt and pepper. Squeeze in the juice from half the lemon and half an orange and stir together.
Place the mackerel fillets in a baking dish, skin-side up. Spoon over the citrus and herb crust to cover both fillets. Set aside for a few mins to marinate.
Scrub the new potatoes and halve or quarter them into equal-sized chunks. When the water is boiling, add the potatoes to the pan. Bring the pan back to a gentle boil. Once it’s there, set a timer for 5 mins.
While the potatoes gently boil, tip the lettuce leaves into a bowl. Halve the cherry tomatoes and add them to the bowl. Trim the roots and top 1cm off the spring onions. Finely slice them, white and green parts, and add them to the bowl.
Your timer should be dinging by now. Slide the dish with the mackerel fillets into the oven and set the timer for 10 mins (this means the potatoes will boil for 15 mins in total).
While the potatoes and mackerel cook, peel and crush the garlic. Pop it in a small bowl and add 1 tsp each of the honey and wholegrain mustard. Squeeze in the juice from the remaining orange half. Add 2 tsp olive oil and a pinch of salt and pepper. Whisk together to make a dressing. Add it the salad bowl and toss to mix.
After 15 mins the potatoes should be tender when pressed with a fork. Drain and let them sit in the colander for a min or 2 to steam dry. Take the mackerel out of the oven. It should flake easily when you press it with a fork.
Arrange the potatoes and salad on 2 warm plates and pop a mackerel fillet on the side of each one. Serve with wedges of the remaining lemon for squeezing.