Preheat the oven to 200°C and put a baking tray in the oven to heat. Lightly oil the inside of a medium soufflé dish (about 1.25 litres). Sprinkle with the breadcrumbs, then tilt the dish and tap around the edge to distribute evenly.
Melt the butter in a saucepan over a medium heat. Sprinkle in the flour, then cook for 1 minute, stirring all the time. Slowly add the milk, whisking constantly, until the sauce comes to the boil and thickens. Simmer for 2 minutes, stirring occasionally, then season with nutmeg and salt and pepper to taste. Meanwhile, gently steam the spinach leaves with just the water clinging to their leaves until dark green. Squeeze out all excess water and season to taste.
Remove the sauce from the heat and cool slightly, then beat in the egg yolks, one at a time. Stir in the spinach leaves. Add the goat's cheese, half the parmesan and the mustard and beat well to mix. Return the pan to the heat and reheat gently, just until the cheese has melted to make a thick, smooth sauce. Set aside to cool.
Whisk the egg whites in a large, clean bowl until stiff peaks form. Whisk one-third of the egg whites into the soufflé mixture to lighten, then use a large metal spoon to fold in the remaining whites.
Spoon the cheese mixture into the soufflé dish and sprinkle with the remaining parmesan. Place on the preheated baking tray and bake for 30–40 minutes until well risen and golden. Serve at once.