Spread the butter onto one side of both slices of bread and place them butter-side down on a plate.
Drizzle the Sriracha over one slice, then layer on the Cheddar, Stilton, kimchi and spring onion. Top with the second slice of bread, butter side face up.
Heat a griddle pan or heavy-based frying pan over a medium heat. Lay the sandwich in the pan and press down with a spatula.
Cook for 3 minutes, then flip over and cook for a further 3 minutes, or until the outside is golden and the cheese has melted. Slice in half and serve.