For the panna cotta:

1. Put half a cup of passion fruit in a small bowl and sprinkle with Knox plain gelatin. Allow to soften for 15 minutes. 
2. Put cream, milk, vanilla, lemon zest and sugar in a heavy-bottomed sauce pan. Heat until mixture just starts to bubble at the edges of the pan. Do not allow to come to a full boil. Remove from heat, cover and let steep for 15 minutes. 
3. Whisk passion fruit and gelatin mixture together and then into the warm milk and cream mixture to dissolve. If your cream mixture has become too cool, you may need to warm it briefly to dissolve the gelatin mixture, just take care not to let it boil as that breaks down the gelatin’s ability to firm up. Whisk in the yogurt and divide into 6 ramekins or decorative glasses. Each portion will be about 3/4 cup.
4. Allow to cool to room temperature then cover each ramekin or glass with a sheet of plastic wrap. Refrigerate for at least 3 hours or overnight.

For the garnish:

Top each panna cotta with fruit from 1/2 passion fruit, including the seeds. Alternatively you can use fresh chopped mango or pineapple.