Add the cubed sweet potato to the baking dish, drizzle with a little olive oil, season with ½ teaspoon sea salt and some ground black pepper and toss to coat evenly. Spread out into an even layer and place into the oven to bake for 20 minutes. After 20 minutes, remove from the oven and turn the oven down to 180°C/356°F.

While the sweet potato bakes, add the eggs to a large mixing bowl along with the milk, wholegrain mustard, remaining salt and some more ground black pepper. Whisk to combine.

Once the sweet potato is baked, remove from the oven and add the baby spinach. Stir through to wilt the spinach. Top the spinach and sweet potatoes with crumbled feta and then pour the egg mixture over the top. Place back into the oven to bake for 20 minutes.

Once the frittata has set, remove from the oven. Slice and serve warm or cold.