Make the dressing. Grind 1/4 toasted pistachios into powder with a blender or food processor. In a small bowl, whisk together the pistachio powder, Dijon, chili flakes, salt, lemon juice and olive oil. Set aside. 

Slice the Brussels sprouts into thin slices or shavings in a food processor. 

In a cooking pan over medium heat, toss the sprout shavings in a tablespoon of oil until softened. 

Mix the salad together in a large bowl. Drizzle the dressing over the sprouts. Add the pistachios and about half of the pecorino. Toss well to combine. Top with the remaining pecorino. Serve immediately. 

Store leftovers in an airtight container in the fridge for up to 4 days.