Heat a little oil in a non-stick frying pan. Add the grated courgettes and cook for a few minutes until they start to colour.
Remove from the oven and leave for a few minutes before turning out onto a wire rack to cool completely.
Bake in a preheated oven 350f for 18 minutes, or until the frittatas are set and golden on the top.
Transfer the egg mixture to a jug, and from there pour the mixture to evenly fill a lightly-oiled 12-cup muffin tin.
Season generously with black pepper and a pinch of salt. Stir the mixture well to combine.
Add the mustard, grated cheese, cumin, mustard, cooked vegetables and chopped chives.
Break the eggs into a large mixing bowl and whisk together.
Transfer the cooked vegetables to a sieve and, using the back of a spoon, gently press the excess liquid out over a bowl or sink. Set the mixture aside and leave to cool.
Add the spinach leaves and cook until wilted down.
Serve with a spoonful of crème fraîche stirred through with chopped chives.