Put the milk and onion in a pan with the bay leaves, if you like, and set over a medium heat. Bring to the boil, reduce the heat and simmer for 2 minutes. Set aside and infuse for 30 minutes.
Pass through a sieve to remove the onion and bay leaves. Return the infused milk to the pan and warm over a low heat.
Melt the butter in another pan, stir in the flour and cook for 2 minutes, stirring constantly. Gradually pour in the milk, whisking until smooth. Stir in 175g cheddar and set aside for 10 minutes. Beat in the egg yolks.
Preheat the oven to 200°C, gas mark 6. Grease 4 x 250ml ramekins.
Put the egg whites in a clean bowl and whisk until stiff peaks form. Gradually fold the cheese mixture into the whites until just incorporated, then divide among the ramekins.
Scatter with the remaining cheese and run your finger around the ramekin rims. Bake for 15-18 minutes until golden and well risen. Serve immediately.