Heat your oven to 200°C/Fan 180°C/Gas 6. Bring a pan of salted water to the boil. Scrub the potatoes and slice them into chips. Add them to the pan and simmer for 5 mins. Drain well.
Tip the chips into a bowl. Add a pinch of sea salt and 1 tbsp oil. Toss to coat. Spread out on a baking tray. Bake in the oven for 30-35 mins, turning halfway through, till golden and tender.
Meanwhile, brush another baking tray with ½ tbsp oil. Lay the salmon fillets on the tray. Sprinkle ½ tsp of the China 5 spice over each fillet and gently rub in. Set aside to marinate for 10 mins.
Spoon the soured cream into a bowl. Finely chop the tarragon leaves. Finely zest the lemon. Add both to the soured cream. Slice the lemon into wedges. Season the soured cream with a little black pepper and stir in ½ tsp of the China 5 spice.
Slide the salmon into the oven on the shelf above the chips. Bake for 10-15 mins till the salmon is cooked through and dark pink.
Break the broccoli into small florets. Roughly chop the parsley leaves. Fill the potato pan with water and bring to the boil.
Chuck the broccoli florets into the pan. Simmer, uncovered, for 5 mins till tender and bright green. Drain well. Serve the salmon with the broccoli, chips and spoonfuls of the herby soured cream sauce.