Heat your oven to 180°C/Fan 160°C/Gas 4. Trim and rinse the leek. Thinly slice it. Slice the Jerusalem artichokes into rounds as thick as a pound coin. Put them into a bowl of water to stop them discolouring. Fill the kettle and boil it.
Melt the butter in a medium pan over a low heat. Add the leeks and the drained artichokes. Cook gently for 15 mins. Stir every so often till the artichokes are tender. Add a splash of hot water if they begin to stick. Cover. Set aside.
While the veg cook, peel and coarsely grate the horseradish. Warm ½ tbsp olive oil in a small pan. Stir in the horseradish. Take off the heat straight away. Set aside to steep for 5 mins.
Rinse the thyme. Pat dry. Strip the leaves from the stalks. Finely grate or pare the zest from the lemon. Halve the lemon. Juice one half (set aside). Mix the thyme and zest into the horseradish with half the lemon juice. Stir in the breadcrumbs. Season.
Line a baking tray with foil. Brush with 1 tsp oil. Pop the salmon on the foil, skin side down. Press the breadcrumb mixture onto the salmon fillets. Bake for 10-15 mins till the fish just flakes.
Rinse and roughly chop the spring greens. Compost the tough cores. Half fill a medium pan with boiling water from the kettle. Pop the spring greens in a steaming basket or colander. Pop it in a pan and cover. Cook for 3-5 mins till tender.
Drizzle the lemon juice over the leek and artichoke mixture. Taste. Season with a little salt and pepper if it needs it.
Serve the artichokes with the baked salmon and spring greens. Use the remaining lemon half for squeezing over the fish.