1. To make the dressing, tip the hazelnuts into a frying pan, toast until golden, then transfer to the small bowl of a food processor. Whizz for 5-10 mins until they make a smooth paste – the nuts will start to release some of their oil and the paste should look like smooth peanut butter. Add the vinegar, maple syrup, season and blend to combine. With the motor running, pour in 150ml water and continue blending until smooth. Can be kept in the fridge for up to 1 week.
2. Cut the apple into really fine matchsticks using a mandolin or sharp knife. Toss in a little of the lemon juice as you go to prevent the apple from turning brown.
3. Put the hazelnuts in a shallow dish. Heat the butter in a frying pan until foaming and nut brown. Add the scallops and cook for 1 min on each side, or to your liking, seasoning as they cook. Remove from the pan and roll each scallop in hazelnuts. If cooking for larger numbers, cook the scallops in batches so that you don’t overcrowd the pan.
To serve, spoon a dollop of the hazelnut dressing in the centre of each plate; use the back of the spoon to spread out the dressing slightly. Toss the watercress in the remaining lemon juice and oil, season and place a pile in the centre of each dressing dollop. Arrange the scallops around the watercress and place piles of apple between each scallop. Drizzle with a little oil and serve immediately