Cut the fennel vertically into long thin slices, rub with olive oil, then grill them in a griddle pan. Keep turning the slices around until they become soft and lightly charred.
Meanwhile, scatter the watercress onto a large platter or divide it between two smaller plates.
Shred the salmon into thin, long strips.
Cut the avocado in half, remove the stone, then cut the flesh into slices or chunks. Immediately squeeze some lemon juice over them.
When the fennel pieces are done, put them on a plate to cool.
When cooled, arrange them on the platter or plates, then add the salmon and avocado slices. Drizzle with oil and season with ground pepper, then drizzle with more lemon juice.