Welsh rarebit with attitude

Melt the butter in a saucepan over medium heat, then stir in the flour and cook for 1 minute.

Gradually whisk in the milk until smooth, then bring to a simmer and cook for 5 minutes, stirring constantly, until thickened.

Remove from the heat and stir in the mustard, cheddar, Gruyère and cayenne pepper until melted and smooth.

Toast the sourdough until golden, then pile the cheese sauce on top and grill until golden and bubbling.