This frittata can be cut into individual portions, wrapped in cling film and tin foil and then frozen for two to three months.
Preheat the oven to 200°C/gas mark 6. Place the squash on a baking tray, then drizzle with 2 tablespoons of oil, the cumin and some salt and pepper and toss to coat. Bake in the oven for 15 to 20 minutes, until cooked through. Remove from the oven and allow to cool slightly.
Meanwhile, heat a splash of oil in a large frying pan set over a medium heat. Add the spinach and cook for 2 minutes, until wilted. Set aside.
Crack the eggs into a large bowl and whisk well, then add the grated courgette, wilted spinach and roasted squash and season to taste. Mix together gently. Heat 2 tablespoons of oil in an ovenproof non-stick frying pan on a medium heat. Once the oil is hot, pour in the egg mixture and cook for 3 to 4 minutes to form a golden crust on the bottom. Place the pan in the oven and bake for 30 to 35 minutes, until the frittata is firm in the middle.
Meanwhile, to make the yogurt topping, place the Greek yogurt in a bowl and stir in the chopped herbs, lemon juice and a pinch of salt and pepper. To serve, slide the frittata out of the pan onto a chopping board and cut into slices. Place a slice of the frittata on a plate with a spoonful of the yogurt on top.