Preheat the oven to 400°F (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 pot 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 a 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.
Cook's tip: You will know the egg whites are stiff enough when you can turn the bowl upside down without them falling out.
Each serving: 18 g protein, 16 g fat of which 8 g saturates, 12 g carbohydrate, 8.5 grams fibre, 278 Calories