In a small saucepan, melt the butter over medium heat. Add the garlic and let it cook for 1-2 minutes, until fragrant and light golden brown.
Whisk in the flour, and cook for 1 minute.
Very slowly, whisk in the cold milk, whisking it to form a smooth paste, then gradually whisking in the rest of the milk. Season with salt, pepper and Italian seasoning. Cook for several minutes, just until slightly thickened, with a consistency similar to warm pudding. Note that the sauce will thicken more as it cools.
Remove the saucepan from the heat and whisk in the Parmesan cheese. Let the sauce cool for a few minutes, or cover and refrigerate for up to 2 days, until needed.
You'll have enough sauce for 1-2 pizzas, depending on how much sauce you like to use. (I only used half the sauce for the photographed pizza.)