Preheat oven to 180 °C , gas mark 2.
Cut the grapefruit in half, score the flesh to taste and sugar lightly.
Whip egg whites with 50 g sugar until stiff, sugar should be almost dissolved. Fold in 50 g sugar and almond kernels (or hazelnuts) very carefully, season with a little lemon juice.
Decoratively pipe the amount onto the grapefruit halves and bake on the highest setting for 5 minutes until the meringue turns brown.
Serve immediately.
Possible refinement: vanilla ice cream scoops, put on each half and form the meringue mixture on top.