Step 1Using a mandoline, shave the fennel against the grain into paper-thin slices. Set aside.
Step 2In a large bowl, whisk together the lemon juice, salt and mustard. Whisk in the olive oil in a slow stream. Add the pepper and whisk again.
Step 3Add the shaved fennel to the bowl, along with the fennel fronds, and toss to coat the fennel well. Adjust the seasoning and set aside.
Step 4On each of 4 plates, arrange one-fourth of the smoked salmon. Next to it, heap one-fourth of the fennel salad, giving it some height on the plate. Garnish with a lemon wedge.