Add the butter and garlic to a large straight-sided pan when melted and the garlic just starts to become golden, stir in the flour, and cook the mixture, until well incorporated.
Add the milk/half and half a little at a time while whisking constantly. It will clump up a bit at first, this is ok! The more milk you add, the clumps will loosen up and dissolve. Let the milk get hot.
Whisk a little of the room temperature lemon juice a little at a time into the bowl of sauce and then whisk in the zest.
Cover, remove from heat and set aside until the pasta is done cooking.
You want it less than al dente because the pasta will continue to cook in the oven.
Strain the water and add it into a 3-quart gratin dish.
Place on the middle shelf for 35 - 40 minutes or until it is golden and bubbly.
Make sure to add the lemon juice at the end.