In a small bowl, mix together the fish sauce, lime juice and sugar. Whisk to dissolve sugar. Set aside.
Peel and cut grapefruit into sections. Store in a bowl. When ready to serve, pour off juice that has accumulated at the bottom of the bowl; reserve for another use.
Add peanuts, shallot, mint and pepper to the grapefruit and toss to combine.
Drizzle the dressing over the top; toss to combine. Taste and if necessary, adjust the balance of salt, sweet and sour. Transfer to a plate lined with lettuce leaves. Serve immediately.