Beat the butter with half of the Natvia. Stir in the desiccated coconut and ½ cup of flour. Press into the bottom of the tray and bake for 15 minutes.
Whisk together the 3 eggs, grapefruit juice, zest, Natvia and remaining flour. Pour over the top of the base. Bake for 12-15 minutes until just set. Cool in the tin and refrigerate. Decorate with slices of grapefruit and sprinkle with desiccated coconut