Photo: Dan Jones
To make the dressing, put the lemon juice, syrup, garlic, mustard and salt into a jar. Add the hazelnut oil and rapeseed oil, close the jar and shake it fiercely to emulsify, then pour into a bowl.
Blanch the asparagus in a pan of boiling water for 1 minute, then drain and refresh in cold water. Chop each asparagus spear in half on a slant and set aside.
Put the unpeeled potatoes in a pot of cold water, cover, and once it comes to the boil, cook for 10-15 minutes or until they can be pierced with a knife. Let them cool slightly, then slice them in half lengthways and heat a griddle pan to very hot. Toss the potatoes in the rapeseed oil and griddle the potatoes flesh-side down until griddle marks appear. You will need to do this in batches.
Pop the hot potatoes into the bowl with the dressing. Add the asparagus and toss.
Put the watercress on a platter and top with the potatoes and asparagus. Scatter with the hazelnuts and some mint leaves.