Heat your oven to 170C/Gas 150C/Gas 5. Peel and grate the garlic and ginger Scrape them into a bowl. Halve the chilli. Finley chop it, flicking out the seeds and white bits for less heat. Finely chop the coriander stalks (keep the leaves for later). Peel and finely slice the shallots. Add them all to the bowl.
Put 1 tbsp of the tikka masala spices and a pinch of salt in a jug. Pour in the coconut milk. Stir to mix well.
Peel the sweet potato and slice into 1/2cm thick slices.
Fill a bowl with cold water. Peel the Jerusalem artichokes and slice them into 1/2 thick slices. Drop them into the water as you slice them (they will brown easily).
Lay some sweet potato slices over the bottom of an 1 ltr ovenproof dish. Scatter over some of the shallot and chilli mix. Drain the Jerusalem artichokes and top with a layer of them. Repeat, till you have used up all the sweet potato, Jerusalem artichokes and shallots.
Stir the coconut mixture again to make sure it is well mixed. Pour over the gratin. Cover the top with a sheet of foil. Bake for 30 mins, taking off the foil for the final 20 mins of baking.
While the gratin bakes, toss the lamb's lettuce with 2 tsp olive oil and a pinch of salt and pepper. Serve the gratin in warm bowls with the lamb's lettuce on the side, garnished with coriander leaves.