Put the cheese, 100g butter and ale in a food processor. Blitz until creamy and well combined. Stir in the mustard, then pack into a large sterilised jar, ramekin or ceramic pot, making sure to eliminate any air pockets.
Melt the remaining butter and leave to sit for a minute to let the fat separate from the milk solids (these will form a milky puddle at the bottom). Pour the clear fat over the cheese, leaving the milk solids in the pan. Place a thyme sprig on top, if using, and chill for a few hours. The sealed cheese will keep for a couple of months in the fridge. Once you’ve cracked the buttery crust, use up within a week.