Prepare and Roast the Squash: Preheat your oven to 200°C (180°C fan). Carefully slice the spaghetti squash in half lengthways and scoop out the seeds. Brush the cut sides with olive oil and season with ½ tsp of salt and the black pepper. Place cut-side down on a baking tray lined with parchment paper. Roast for 30-40 minutes, or until the flesh is tender and can be easily pierced with a fork.

Scrape the Squash Strands: Once roasted, let the squash cool for about 10 minutes until it's safe to handle. Use a fork to scrape the flesh from the peel; it will naturally separate into long strands. Place the strands in a colander set over a bowl and gently press to release any excess moisture. This is a crucial step to avoid a watery gratin.

Start the Béchamel Sauce: While the squash cools, melt 50g of butter in a medium saucepan over medium heat. Add the minced garlic and cook for about a minute until fragrant, but not browned. For a detailed guide on the base of this sauce, BBC Good Food has an excellent article on how to make a classic béchamel sauce.

Make the Roux: Whisk the plain flour into the melted butter to form a smooth paste, known as a roux. Cook, stirring constantly, for 1-2 minutes. This cooks out the raw flour taste.

Add the Milk: Gradually pour in the warm milk, a little at a time, whisking continuously to prevent lumps from forming. Once all the milk is incorporated, bring the sauce to a gentle simmer and cook for 5-7 minutes, stirring often, until it has thickened enough to coat the back of a spoon.

Stir in the Cheeses: Remove the saucepan from the heat. I find that this prevents the cheese from becoming grainy or splitting. Add the grated Gruyère, grated cheddar, grated nutmeg, and the remaining ½ tsp of salt. Stir until the cheeses are completely melted and the sauce is smooth and velvety.

Combine and Assemble: Lower the oven temperature to 180°C (160°C fan). In a large bowl, gently combine the spaghetti squash strands with the cheese sauce, ensuring all the strands are well-coated. Transfer the mixture to a 2-litre ovenproof baking dish and spread it out evenly.

Prepare the Topping: In a small bowl, mix the breadcrumbs, grated Parmesan, and the melted tablespoon of butter together until the crumbs are evenly moistened.

Bake the Gratin: Sprinkle the breadcrumb mixture evenly over the squash. Bake for 20-25 minutes, or until the topping 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.