Sprinkle the salmon fillet lightly with rock salt and refrigerate for 20 minutes.
Wash the salt off the salmon and pat the fillet dry.
Whisk chillies, grapefruit juice, lime zest, lime juice, and sugar in a bowl.
Add the chopped coriander to the dressing and toss to combine.
Slice the salmon at an angle into pieces 2-3mm thick.
Arrange four salmon slices on each plate and top with most of the dressing.
Let the salmon marinate for 30 minutes.
Soak sliced fennel in iced water for 3 minutes then drain thoroughly.
Toss fennel and salad leaves with the remaining dressing.
Mound the salad on top of the salmon and serve.