Preheat oven to 180°C (160°C fan) if finishing in oven.
In a non-stick oven-safe frying pan, heat olive oil.
Sauté the onion for 2–3 minutes until soft.
Add courgette and asparagus (if using) and cook for 2–3 minutes more.
Add spinach and cook just until wilted..
Season with salt and pepper, piment d'espelette
In a bowl, whisk eggs with milk, herbs, and a pinch of salt. Pour over vegetables in the pan. Stir lightly, or tilt to distribute.
Sprinkle cheese evenly over the top. sprinkle a little more piment d'espelette
Cook gently on the hob for 2–3 minutes until the edges begin to set.
Transfer to the oven and bake for 8–10 minutes, or until just set and lightly golden.
Serve with freshly baked baguette and sea-salt butter