Preheat your oven to 200°C (180°C fan/gas mark 6). Cook the macaroni in a large pot of boiling salted water for 8-10 minutes until al dente, then drain and set aside.
In a medium saucepan, melt the butter over medium heat. Add the flour and whisk continuously for about 2 minutes until a smooth roux forms.
Gradually whisk in the milk and double cream, stirring constantly until the sauce thickens and begins to bubble, about 5 minutes. Season with mustard powder, a pinch of nutmeg, salt, and pepper.
Remove the sauce from the heat and stir in the grated cheddar and Gruyère cheeses until melted and smooth. Combine the cheese sauce with the cooked macaroni.
Transfer the macaroni mixture to a baking dish. In a separate pan, heat a little butter and sauté the garlic until fragrant. Mix the breadcrumbs with the sautéed garlic, then sprinkle evenly over the macaroni.
Bake in the preheated oven for 20-25 minutes until the top is golden and crispy. Serve immediately, garnished with fresh herbs if desired.
