Place the water, milk and butter into a pan over a medium-high heat. Let the butter melt and allow the mix to just come up to the boil. Immediately reduce the heat to medium, tip all the flour into the pan and mix with a wooden spoon to form a paste.
After 1-2 minutes of cooking, the mix will start to furr at the bottom of the pan, at this point remove from the heat to cool for a minute.
Gradually add the egg to the paste, beating thoroughly after each addition. Once all the egg is incorporated, stir in 170g grated cheddar, the mustard, thyme and a good pinch of salt and pepper.
You can make this fritter paste ahead of time; just refrigerate it until you are ready to use.
Heat about 400ml of flavourless oil (such as sunflower oil or rapeseed oil, so long as it is not cold-pressed) in a saucepan to 180C. Dollop teaspoons of the fritter batter into the hot oil, a few at a time, and fry for a few minutes until golden brown.
Remove with a slotted spoon onto kitchen towel, then sprinkle with flaky sea salt.
Serve them piled up on a plate with an extra sprinkling of finely grated Westcombe Cheddar to finish.