Blanch tomatoes, remove skin. Peel the grapefruit like an apple, cut away the thin skin. Remove the fruit fillets, blend with the tomatoes in a hand blender, pass through a sieve.
Finely chop the leaves of 2 mint stalks, add to the tomato mixture with Pernod and strain through a sieve.
Season with salt and cayenne pepper. Freeze in a sorbetiere (ice cream maker) or fill into metal bowls and put into the freezer compartment for at least 3 hours.
Beat with a blender, stir in stiff egg whites. Freeze sorbet one more time for 1 hour. Serve in chilled glasses with a splash of red grapefruit juice and mint.