Begin by grating 250 grams of mature cheddar cheese and set it aside.

In a saucepan, melt 50 grams of butter, then add 50 grams of plain flour. Cook gently while stirring continuously until the mixture turns lightly golden and releases a nutty aroma.

Gradually add 150 milliliters of ale, a little at a time, stirring constantly until the mixture is smooth.

Slowly incorporate 150 milliliters of milk, continuing to stir until everything is fully combined.

Mix in one heaped teaspoon of mustard, one teaspoon of Worcestershire sauce, and a small pinch of white pepper.

Allow the sauce to simmer gently for 5 to 10 minutes, stirring continuously until it thickens.

Remove the saucepan from the heat and gradually stir in the grated cheese, adding it a handful at a time and ensuring it melts completely before adding more.

Transfer the cheese mixture to a tray or bowl, cover the surface with parchment paper, and refrigerate until it firms up.

For the topping, slice 2 leeks lengthwise, wash them thoroughly, and then slice them thinly.

In a frying pan over medium heat, melt 30 grams of butter and cook the leeks with a pinch of salt until they become soft.

Lightly toast slices of sourdough bread under the grill until they are golden.

To assemble, spread a layer of the buttered leeks over the toasted bread. Add sliced ham, followed by a bit more leeks.

Take the chilled rarebit mixture and use a spoon to seal the edges, then place more of the mixture on top until it is generously covered.

Place the assembled dish under a hot grill and cook until the top is golden, bubbling, and lightly charred.

For added flavor, drizzle a splash of Worcestershire sauce on top, then remove from the grill, allow to cool slightly, slice, and serve immediately.