Prepare the Garlic: First, ensure your garlic is very finely minced. You can do this with a sharp knife or use a garlic press. The smaller the pieces, the more evenly the flavour will infuse into the sauce. Set aside.

Melt the Butter: Place a medium-sized saucepan over a medium-low heat and add the 50g of unsalted butter. Allow it to melt completely without browning.

Sauté the Garlic: Add the minced garlic to the melted butter. Sauté gently for 1-2 minutes, stirring continuously with a wooden spoon or spatula. You want the garlic to become fragrant and soft, but it's crucial that it doesn't take on any colour. If it starts to brown, it will turn bitter.

Create the Roux: Sprinkle the 40g of plain flour over the garlic butter. Immediately begin to whisk it all together to form a thick paste. Cook this roux for about 60-90 seconds, still stirring. This step cooks out the raw flour taste.

Add the Milk Gradually: Now, switch from a spoon to a whisk. While whisking constantly, start adding the 400ml of room-temperature milk a small splash at a time. I find that incorporating the first 100ml very slowly is the absolute key to a smooth sauce. Once the initial paste has loosened into a thick slurry, you can add the rest of the milk in a slow, steady stream, whisking all the while.

Thicken the Sauce: Increase the heat to medium and bring the sauce to a gentle simmer. Continue to whisk often, scraping the bottom and corners of the pan, for about 5-7 minutes. The sauce will noticeably thicken to the consistency of single cream.

Add the Cream and Cheese: Reduce the heat to low. Pour in the 150ml of double cream and add the 50g of grated Parmesan cheese. Stir gently until the cheese has completely melted and the sauce is smooth and glossy. Do not let the sauce boil after adding the cheese, as this can cause it to split.

Season and Serve: Turn off the heat. Season the sauce with the salt, black pepper, and a tiny pinch of nutmeg. The nutmeg might sound odd, but it wonderfully enhances the creamy flavours. Finally, stir in the finely chopped fresh parsley. Give it a final taste and adjust seasoning if needed before serving warm.