1. The day before, remove the outer leaves of the fennel, wash it, and grate it. Mix it with the mascarpone. Season lightly with salt and generously with pepper.
2. On your work surface, place a slice of salmon, spread some of the mixture on top, cover with a second slice of salmon, and then the mixture. Roll it all up and wrap it in plastic wrap.
3. Repeat with the other 2 slices of salmon and refrigerate until the next day.