Preheat the oven to 190°C/fan 170°C/gas mark 5. Toss the sweet potato in a little rapeseed oil and season with black pepper. Roast for about 20-25 minutes or until cooked through. Set aside.
Defrost the peas. Wilt the spinach in a frying pan with a little rapeseed oil then roughly chop and set aside. Finely chop the mint leaves.
Whisk the eggs and milk together in a jug and season. Crumble in half the feta, half the mint, all the spinach and the lemon zest.
Gently heat 1 tbsp rapeseed oil in an oven proof frying pan and pour in the egg mixture. Evenly distribute the peas over the surface and cook for around 5 minutes until the bottom starts to set, then sprinkle over the remaining feta. Arrange the sweet potato pieces on top and place in the oven to cook for a further 10-15 minutes at 160°C/fan 140°C/gas mark 3.
Meanwhile, combine the remaining mint, 1-2tbsp rapeseed oil, the lemon juice (to taste) and some seasoning to create a drizzle.
When the frittata is ready it should be golden brown on top with a slight wobble in the centre. Leave to cool a little then slice up and serve. Finish with a drizzle of the mint oil and a side salad.
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