Begin by making the pastry. Place the flour and butter in a food processor and process until the mixture resembles bread crumbs. Add the egg yolk and water and pulse until the mixture begins to come together to form a ball. Remove from the food processor and place onto plastic wrap and shape into a disc. Refrigerate for 30 minutes.
Remove the pastry from the refrigerator and roll out the pastry on a lightly floured bench or silicone baking mat until approximately 5mm thick. Line a 8 small loose bottomed tart tin with the pastry, making sure to press into the sides, and trim the excess. Refrigerate for 15 minutes.
Preheat the oven to 180 celsius (355 Fahrenheit) and line the tart with baking paper and fill with baking weights or beans. Bake for 10 minutes before removing the baking paper and weights and baking for a further 5 minutes or until the base is lightly golden. Set aside to cool.
In a bowl add the eggs and egg yolk along with the yoghurt and milk and whisk to combine. Add the harissa and season to taste.
Crumble the feta into the bottom of each tart and top with tomato slices, layering each slice in a circular pattern (about 4 or 5 slices, depending on the size of your tomatoes). Pour the egg mixture over the top and sprinkle with the thyme leaves. Bake for 8 to 10 minutes or until the filling is just set.
If you don't feel up to making your own pastry use store bought frozen pastry such as a Careme. If using a store bought Harissa you might need to adjust the quantity of Harissa used. Start with 2 tsp and adjust as necessary. Danish feta is a creamy style of feta, which I prefer, but feel free to use whichever feta you have on hand or prefer.