Preheat the oven to 200C/400F/Gas 6.
Place the Stilton into a small ovenproof frying pan over a low heat and melt gently. Add the mustard and take the pan off the heat. Stir in the egg yolks.
Place the egg whites into a clean bowl and whisk until stiff peaks form when the whisk is removed. Fold the egg whites into the cheese mixture.
Transfer the pan to the oven and cook for 6-7 minutes, or until the soufflé is set and golden-brown.
For the pesto, place the basil, parsley and oil into a small food processor and blend until smooth, then season, to taste, with salt and freshly ground black pepper.
For the spinach, melt the butter in a frying pan, add the spinach and cook for 2-3 minutes until wilted.
To serve, place the spinach onto a plate with the soufflé on top and the pesto drizzled over.