Preheat your oven to 200°C/Fan 180°C/Gas 6. Scrub the pink fir apple potatoes and chop them into bite-sized chunks. Peel the red onions and slice them into 6-8 wedges each. Scatter the potatoes and onions into a roasting tin and toss with 1 tbsp oil and a pinch of salt and pepper. Slide the tin into the oven for 25 mins.

Fill a pan with water and add a pinch of salt. Pop the pan on the hob and bring to the boil. Trim any woody ends off the purple sprouting broccoli and cut any larger pieces on half so the florets are all about the same size.

When the water in the pan is boiling, drop in the watercress. Simmer for 30 secs, then lift it out of the pan with a slotted spoon or tongs (keep the boiling water for later). Squeeze any excess water from the cress and pop it into a food processor.

Finely grate the garlic clove into the processor and juice in the lemon. Add 1 tsp Dijon mustard, 1 tbsp oil and a little salt and pepper. Blend well, till finely chopped. No food processor? Just chop the watercress and garlic as finely as possible before stirring in the other ingredients.

When the veg have roasted for 25 mins, remove the tin from the oven. Turn the potatoes and onions over. Rub the salmon fillets with 1 tsp oil and a little salt and pepper. Nestle the fillets into the potatoes and onions, skin-side up. Slide the tin back into the oven to roast for a further 12-15 mins, till the fish is flaky and cooked through.

Meanwhile, add the purple sprouting broccoli to the pan with the boiling water. Simmer for 4-5 mins, till tender. Drain and shake dry.

Divide the salmon, potatoes and onions between a couple of plates. Spoon over the watercress sauce and serve with the purple sprouting broccoli on the side.