Preheat the oven to Gas Mark 6, 200°C (400°F). When the oven is hot, add the cherry tomatoes, drizzled with a little oil and sprinkled with salt and pepper. (These will take about 20 minutes to cook and will be ready by the time the rösti are).
Wilt the spinach by pouring boiling hot water over it in a colander in the sink.
Remove the excess water by placing an empty bowl on top of the spinach and pushing the liquid through the colander (the rösti will cook much better if all excess water is removed).
Remove the spinach from the colander and allow to cool.
Peel and grate the potatoes and place them in a bowl. Add the egg, flour, scallions, nutmeg, chilli flakes, salt and pepper and stir well.
Add the wilted and squeezed spinach and stir again. Finally add the crumbled feta and stir briefly to incorporate.
Heat the oil until hot in a large frying pan (this is important as you really want the rösti to be crisp).
Spoon the mix into the pan – forming shapes about 10 cm wide. After about 3 minutes, carefully flip each rösti.
Cook for another 3 minutes, then transfer to a large oven tray to finish cooking for a further 10 minutes (you may need to do this in batches).
Serve the rösti with the roasted cherry tomatoes on top.