Make the vinaigrette: Put the lemon juice, shallot, mustard, salt, sugar and mint in a blender and pulse briefly to combine. With the motor running, drizzle in the olive oil until it is well combined.

Shave the fennel into thin slices: Using a mandoline, shave the fennel into ⅛-inch slices starting from the bottom of the bulb. Don't worry about coring the fennel bulb, it's unnecessary. If you don’t have a mandoline, slice the bulb as thin as you can. Chop some of the fennel fronds as well to toss in with the salad.

Marinate fennel with vinaigrette: Toss with the fennel and marinate for at least an hour. Serve this salad either cold or at room temperature.