Dice off the coarse green tough part of the leek (save for making stock if you wish). Keep the root intact and slice in half lengthways. Clean out any dirt under the tap or in a bowl of cold water then thoroughly dry. Trim off the root then season the insides with a good pinch of salt and pepper.

Add 1 tbsp butter and 1 tbsp olive oil to a large pan over low-medium heat. Add leeks (work in two batches, or as many as you can fit) seasoned/flesh side down and gently fry until soft and buttery. Discard any papery outer layers that detach. Really take the time to ensure they soften right through. If your leeks are quite thick consider adding a splash of stock or water, putting on the lid and allowing them to steam through. That or quarter instead of halving them. Place to one side and repeat with second batch, adding a splash more oil if needed.

Turn heat to a medium and melt in 2 tbsp butter. Stir in 2 tbsp flour to form a paste, then very gradually begin adding in 2 cups/500ml milk, whisking as you go to avoid any lumps forming. Once thickened, add in 1 tbsp dijon mustard, a good few pinches of nutmeg and salt & pepper to taste. Turn off heat then add in 1 cup/100g Gruyere and 1 cup/100g Cheddar and stir until smooth.

Spread a spoonful of cheese sauce in the base of a 9"x9" baking dish. Add 6 leek halves cut side up, pour over sauce, add the remaining leeks on top then pour over the rest of the sauce. Finish with your remaining cheddar and bake in the oven at 200C/390F for 20-25mins or until golden and bubbly.

Allow to rest for 5 or so mins until it cools down, then slice right down the middle. Serve up and enjoy!