Butter 8 ramekins or one 4 cup/1-litre baking dish or soufflé dish and dust, if you like, with grated Parmesan. Heat the oven to 400°F/200°C. Bring a large kettle to the boil.
Heat the milk with the bay leaf, cover, and leave to infuse off the heat 10 minutes. Remove the bay leaf.
In a small saucepan, melt the butter, whisk in the flour, and cook the mixture 1 minutes. Whisk in the milk and cook, stirring constantly, until the mixture coats a spoon, a matter of minutes.
Whisk the yolks in a bowl and whisk in the hot béchamel. Remove from the heat and whisk in the vegetable purée and cheese. Season very highly.
Beat the whites to stiff peaks. Stir some of the whites into the vegetable base to loosen it up. Then pour the vegetable mixture over the remaining whites and fold together gently. Spoon into the baking dishes. Run a finger around the edge so they’ll rise evenly.
Place the ramekins in the bain-marie. Pour water into the bain-marie. Bake until puffed high and golden on top, but still creamy in the centre, 15 to 25 minutes. Remove from the water bath and serve immediately.
Or, let the soufflés sink down in the ramekins. Heat the oven to 425ºF/220ºC. Flip the soufflés out into a gratin dish. Pour over a little cream mixed with left-over béchamel. Sprinkle with herbs and Parmesan. Bake five to seven minutes until they puff up again and the sauce bubbles. Serve.