Prepare and Roast the Squash: Preheat your oven to 200°C (180°C fan). Carefully slice the spaghetti squash in half lengthwise and scoop out the seeds. Brush the cut surfaces with olive oil and season with salt and pepper. Place the halves cut-side down on a baking tray lined with parchment paper.

Bake the Squash: Roast for 30-40 minutes, or until the flesh is tender and can be easily pierced with a fork. The exact time will depend on the size of your squash. Once cooked, remove from the oven and let it cool slightly.

Make the Roux: While the squash is roasting, melt 50g of butter in a medium saucepan over a medium heat. Whisk in the plain flour and cook for 1-2 minutes, stirring constantly, to create a paste (this is called a roux).

Create the Béchamel Sauce: Gradually pour in the warm milk, a little at a time, whisking continuously to ensure a smooth, lump-free sauce. Bring the sauce to a gentle simmer and cook for 5-7 minutes, until it has thickened enough to coat the back of a spoon.

Add the Flavour: Remove the sauce from the heat. Stir in the minced garlic, ground nutmeg, Dijon mustard, and most of the grated Cheddar and Gruyère cheeses (reserving a small handful of each for the top). Stir until the cheese is gloriously melty and the sauce is smooth. Season generously with salt and pepper to taste.

Prepare the Squash Strands: Once the squash is cool enough to handle, use a fork to scrape the flesh from the skin. The flesh will naturally separate into long, spaghetti-like strands. I find that holding the squash half steady with an oven glove makes this step much easier. You should have about 800-900g of cooked squash strands.

Combine and Assemble: Transfer the squash strands to a large mixing bowl. Pour the cheese sauce over the squash and gently toss to combine everything thoroughly. Tip the mixture into a 2-litre ovenproof baking dish and spread it out evenly.

Add the Topping: In a small bowl, mix together the panko breadcrumbs, grated Parmesan, and chopped parsley. Drizzle with the melted butter and toss to combine. Sprinkle this mixture evenly over the squash. Top with the reserved Cheddar and Gruyère cheese.

Bake Until Golden: Bake at 190°C (170°C fan) for 20-25 minutes, or until the top is golden brown and the sauce is bubbling around the edges.

Rest and Serve: Let the gratin rest for at least 10 minutes before serving. This allows the sauce to set slightly, making it easier to serve. Garnish with a little extra fresh parsley if you wish.