Preheat the oven to 200°C/Gas Mark 6/180°C fan. If using, infuse the saffron in a couple of tablespoons of warm water.
Heat the oil in a small (24cm) frying pan.
Fry the onions, garlic and cumin over a low heat until the onions are soft for about 10 minutes. Add the pre-boiled potatoes and turn the heat up, mix to incorporate, then add the tomatoes (and saffron water if using). Season well and cook for a further 5 minutes, stirring occasionally (add a splash of water if the mixture is drying up and sticking). Season to taste.
Using a wooden spoon make 4 shallow wells in the mix and break the eggs into them.
Place the pan in the oven and bake until the white of the eggs are cooked and the yolk still looks a little runny (for about 5-10 minutes).
Let it sit for a few minutes, then sprinkle with coriander, yogurt and chilli flakes.