Preheat the oven to 180C/365F/Gas 4.
Place the milk, onion and peppercorns into a pan over a medium heat and bring to scalding point (but do not boil). Strain the milk into a bowl and discard the onion and peppercorns.
Meanwhile, melt the butter in a clean pan over a medium heat. Add the flour and stir well for one minute to create a paste.
Gradually add the heated milk, a little at a time, waiting for the flour to bubble before adding more milk and stirring between additions. Keep adding to create a smooth sauce.
Remove the pan from the heat, then add the egg yolks and thyme and whisk to combine.
Add 75g/2½oz of the goats' cheese, season with salt and freshly ground black pepper and stir well.
Whisk the egg whites until stiff peaks form when the whisk is removed.
Add a small spoonful of the egg whites to the cheese sauce and mix well.
Gradually add generous spoonfuls of the egg whites to the cheese sauce, gently folding the whites into the sauce, until the whites are completely combined with the sauce.
Butter and flour four 150g/5oz ramekins and divide the cheese mixture among them.
If cooking to serve immediately, sprinkle the soufflés with the remaining goats' cheese; if planning to store and reheat, reserve the cheese for later.
Transfer the soufflés to an ovenproof dish and add enough water to the dish to come a third of the way up the ramekins.
Carefully place into the oven to bake for 15-18 minutes, or until risen.
The soufflés can be cooled and stored in the fridge, or even frozen, to be cooked later. If reheating, defrost or remove from the fridge, top with the remaining cheese and place into an oven preheated to 180C/365F/Gas 4 and bake for 8-10 minutes, or until heated through.
To serve, place the soufflés onto plates and place a handful of watercress alongside.
