Homity Pie with Cornish new potatoes and Barber's 1833 cheddar

Ingredients

• 400g plain flour
• 350g frozen butter (put in the freezer at least two hours before)
• 1/2 teaspoon salt
• 2 teaspoons English mustard
• Around 100ml cold water
• 3 potatoes, about 500g, cut into slice 1/2 cm thick
• 150g Barber 1833 cheddar
• 1 small onion, about 75g, finely chopped
• 1 desert spoon English mustard
• 4 sprigs of thyme, leaves removed and stalks discarded