Oven on, water on
Preheat the oven to 200°C and lightly coat a baking dish with nonstick cooking spray. Fill a medium pot with water, bring it to a boil over high heat, and add a pinch of salt.
Three cheeses, a proper roux, and macaroni seasoned in its own right before the sauce ever touches it. Baked until the top browns and the sauce comes up round the edges.
Season the noodles, not just the sauce
The macaroni gets butter, pepper and garlic salt of its own after draining. Skip that and the dish tastes like sauce poured over plain pasta, which is exactly what it is trying not to be.
Preheat the oven to 200°C and lightly coat a baking dish with nonstick cooking spray. Fill a medium pot with water, bring it to a boil over high heat, and add a pinch of salt.
In a second medium pot over medium-low heat, melt most of the butter, keeping a little back for the macaroni. Add the flour, some of the black pepper, some of the garlic salt, the mustard powder and the cayenne, and whisk constantly until everything is incorporated and the flour no longer tastes floury.
Whisking constantly, very slowly drizzle in the milk and most of the half-and-half, continuing to whisk until the liquid is fully incorporated and the mixture begins to bubble gently.
Continue to cook, stirring constantly, until the mixture has thickened just enough to fully coat the back of a spoon. Remove from the heat, add all the extra-sharp cheddar, most of the medium cheddar and a little of the Monterey Jack, and stir until the cheeses have melted. Keep the rest of the medium cheddar and the Jack back for the top.
Be very careful not to scorch the milk, or the whole dish will have a bitter, burned flavour.
Meanwhile, add the macaroni to the boiling water and cook until al dente — about a minute less than the time on the packet, because it still has the oven to go. Drain it and transfer it to a large bowl.
Add the butter you kept back and the rest of the black pepper and garlic salt to the drained macaroni, and mix well. This is the step that separates the dish from sauce poured over plain pasta.
Add the cheese sauce to the seasoned macaroni and mix thoroughly. Transfer the mixture to the prepared baking dish and level the surface. Sprinkle it evenly with the medium cheddar and Monterey Jack you kept back.
Bake until the sauce is bubbling and the top is golden brown. Serve hot or warm.