Ingredients

Equipment Needed

• •Heavy-bottomed saucepan
• •Grill or overhead broiler
• •Sturdy baking tray for grilling the toast

Instructions

1. 1

   Soften the leeks

   Melt the butter in a heavy pan over a low heat. Add the sliced leeks and a pinch of salt. Stir them through the butter, cover with a lid, and let them sweat gently for eight to ten minutes, stirring now and again. You want them completely soft and silky, almost melting, with no colour at all. If they start to catch, add a splash of water and turn the heat down. When they're done, they should smell sweet and gentle, nothing sharp left. Tip them onto a plate and set aside.

   Wash leeks well before slicing. Split them lengthways and fan the layers under running water. Grit hides where you least expect it, and nothing ruins a good rarebit like crunching on sand.
2. 2

   Build the rarebit mixture

   In the same pan, add the flour and stir it into the remaining butter for half a minute. Pour in the ale. It will fizz and spit. Let it bubble for a minute until it thickens into something that coats the back of a spoon. Take the pan off the heat and add the crumbled Caerphilly, the mustard, and the Worcestershire sauce. Stir until the cheese has mostly melted into a thick, rough paste. It won't be perfectly smooth because Caerphilly is crumbly by nature. That's fine. That's what gives it character. Let it cool for a minute, then stir in the egg yolk and a good grinding of black pepper.

   Take the pan off the heat before adding the cheese. If it's too hot, the Caerphilly can turn grainy. Gentle warmth and patience is all it needs.
3. 3

   Fold in the leeks

   Add the softened leeks to the cheese mixture and fold them through. Taste it. The leeks should be sweet against the tang of the Caerphilly, with a low hum of mustard and the ale lingering somewhere behind. Adjust the seasoning. More pepper, perhaps. More mustard if you like heat. Trust your instincts here. This is your rarebit.

4. 4

   Toast and grill

   Heat your grill to high. Toast the bread on one side only, directly under the grill. Turn the slices over and pile the rarebit mixture generously onto the untoasted side, spreading it right to the edges so the bread doesn't burn at the corners. Return to the grill and cook until the top is bubbling and blistered, with patches of deep gold and brown. This takes three to four minutes, but watch it. The line between golden and charred is narrow, and your nose will tell you when it's crossed. Serve immediately, on warm plates if you've remembered to heat them.

   Spreading the mixture to the very edges matters. Exposed bread under a hot grill burns quickly. The cheese protects whatever it covers.

Chef Tips

• •Caerphilly varies. A younger one, chalky and fresh, melts differently from one that's been aged a few months. Either works, but the younger cheese gives a lighter, more tangy rarebit. Ask your cheesemonger what they'd recommend for cooking.
• •The ale matters more than you'd think. Use something you'd drink: a Welsh bitter or a decent pale ale. Nothing too hoppy, nothing too dark. The ale is a background note, not the lead. A stout would bully the cheese.
• •If you're feeding someone who doesn't eat meat but worries about Worcestershire sauce, leave it out. A squeeze of lemon juice in its place gives the same brightness. The dish loses nothing.
• •Leftovers of the rarebit mixture keep in the fridge for a day, covered. Spread onto toast straight from the fridge and grill as before. It's arguably better the second time, once the flavours have had the night to settle.

Advance Preparation

• •The rarebit mixture can be made several hours ahead and refrigerated. Bring it back to room temperature before spreading on the toast, or it won't spread easily.
• •The leeks can be softened earlier in the day and folded into the cheese mixture when you're ready to cook. The assembly then takes five minutes.

Frequently Asked Questions

Nutrition Information

1 serving (about 200g)