In a food processor, purée the garlic and onion into a paste. Smear over the block of Wookey Hole Cheddar and allow to marinate in a bag in the fridge overnight.
The next day, remove the cheese from the fridge. Scrape off and discard any purée on the cheese. Grate the cheese.
Preheat the oven to 180 C / Gas 4. Grease four individual ramekins.
In a saucepan over a medium heat, melt the butter. Stir in the flour and mustard powder and cook for 2 to 3 minutes until straw coloured. Whisking constantly, gradually add the milk and cook until thickened. Reduce the heat to low and add the cheese, stirring until melted.
Beat the egg yolks until thick and creamy then gradually add in about a third of the cheese sauce, whisking constantly. Add the yolk mixture to the cheese sauce and cool slightly. In a separate bowl, whisk the egg whites until soft peaks form then gently fold into the mixture. Divide equally among the four ramekins.
Bake in the preheated oven for 20 to 25 minutes until risen and golden brown. Serve immediately.