Preheat the oven to 190 degrees. Scrub the sweet potatoes, then rub them with a little salt, pepper and olive oil and place them on a baking tray. Roast for 50 minutes.
Put a non-stick pan on a medium heat, add some olive oil and the spinach, some salt and pepper and a good pinch of nutmeg. Heat gently until the spinach has wilted and all the water has evaporated.
Whilst the spinach is doing its thing, crack the eggs into a mixing bowl, season and grate in half the cheddar and half the feta, crumbled.
Remove the spinach from the heat and pour in the egg mixture and give it a really good stir. Rip in chunks of sweet potato (with or without skin), then break over the remaining feta and cheddar.
Put the pan straight into the oven and cook for 15-20 minutes until fluffy and golden. Serve with a lemony-dressed simple salad.