Trim the hard ends of the Brussels sprouts and peel off the outer leaves. Halve them lengthwise. Be sure to keep any loose leaves, as they will separate during cooking and become crispy like chips; that is the best part of this dish.
Dice the pancetta and place it in a cold frying pan. Only then turn the heat to medium. This allows the fat to render gradually and the bacon to crisp up evenly, instead of burning on the outside while remaining chewy inside. Scoop the cubes out after six to eight minutes and drain on paper towels.
Add a tablespoon of olive oil to the remaining fat and place the Brussels sprouts cut-side down in the pan. Leave them for four minutes without stirring until the cut side is deep brown, then shake the pan and fry for another four minutes until tender-crisp.
Dry-toast the hazelnuts in a second pan until the skins crack, rub them in a kitchen towel, and roughly chop them.
Whisk the mustard with the apple cider vinegar, honey, finely chopped shallot, the remaining olive oil, salt, and pepper to make a dressing.
Mix the warm Brussels sprouts with the lamb's lettuce and the dressing, toss in the pancetta, and finish with the hazelnuts and shaved aged cheese.