Preheat your oven to 190°C (fan). Using a mandolin or sharp knife, slice the potatoes into thin rounds. Rinse them in ice-cold water and pat dry thoroughly.
In a small bowl, whisk together the egg yolk and milk to create the egg wash.
Line a baking tray with parchment paper. Arrange 1 potato slice in the centre and 6 slices around it, slightly overlapping, to form 6 "flowers." Brush each generously with the egg wash, then sprinkle with Parmesan and thyme leaves. Bake for 15 minutes until golden but still flexible.
Once cooled enough to handle, gently transfer the potato circles into a muffin tin, pressing them upside down to form cups.
Divide half of the mozzarella equally between the cups, add cherry tomato halves and 2 slices of leek to each. Top with the remaining mozzarella and a sprinkle of fresh thyme.
In a separate bowl, whisk together the egg, milk, and a pinch of salt. Pour the mixture evenly into the potato cups.
Turn off the fan and bake for 15-18 minutes, or until the filling is set and the cheese has melted.
Let the potato cups cool in the muffin tin for 1o minutes before carefully removing them with a spatula. Garnish with fresh thyme leaves and enjoy!
