I found this recipe on Food52 after having something just like it at Pizzeria Da Lupo.
It is really tasty in a tangy but not bitter sort of way, and pretty easy if you have a food processor to slice the Brussels sprouts.
* 1/2 small red onion
* Juice of 1 lemon (~3-4 Tbsp)
* 1 teaspoon honey
* 1 teaspoon whole grain mustard
* Salt and freshly ground black pepper
* 1 tablespoon olive oil
* 3 cups Brussels sprouts (use larger sprouts if possible)
* 1/2 cup finely grated pecorino romano
1. Soak the onion slices in a small bowl of cold water for 15 to 20 minutes while you prepare the rest of the ingredients.
2. In a small bowl, whisk together the lemon juice, honey, mustard and a pinch of salt and pepper. Whisk in the olive oil until the dressing is emulsified. Set aside.
3. Trim all of the Brussels sprouts, cutting off any bruised outer leaves and slicing off a good portion of the hard root end. Using a mandoline (or a food processor!), shave the sprouts one at a time. When you're done, use your fingers to gently separate the leaves so that the shredded sprouts resemble a very fine slaw.
4. Put the sprouts in a serving bowl and toss gently with the onions (which you've now drained) and the dressing. Fold in the pecorino, taste and adjust seasonings if necessary.