Rinse the arugula and dry well, roughly chop or tear. Place the dried arugula in a bowl and set aside.
Place the hazelnut oil, vinegar, mustard, sugar, and cherries in a small saucepan over medium heat and stir with a whisk until warm (not boiling - 3 minutes or so). Pour over the arugula, sprinkle with a little salt, and toss to coat.
Place the arugula on a plate and top with the duck slices (you can remove the duck fat beforehand and save for another purpose). With a vegetable peeler, peel thin slices of the cheddar over the salad. Top with the chopped hazelnuts and a little extra dressing, adding any cherries to the plate that did not cling to the greens. Sprinkle everything with a little sea salt.