Cut the bacon into the thinnest strips you can manage. This is easiest if you freeze the bacon first. Throw the bacon into a medium pot and put it on medium heat. Fry until crispy, about 6 to 8 minutes. Remove the bacon to a paper-towel lined plate to drain.

If there's less than 3 or 4 tbsp bacon grease remaining in the pot, add a small knob of butter to top it up. Add the shallot and garlic and fry for 1 or 2 minutes, just until softened. Don't let the garlic brown. Add the flour and stir to coat. Let cook for 3 minutes, stirring frequently, to get the raw flour taste out.

While whisking, add 1 cup of the milk to the pot. Once it's warmed up and started to thicken slightly add the rest of the milk. Keep whisking until the sauce is the thickness of evaporated milk. This will take about 10 minutes.

Break the cheese up into small chunks and whisk it a little at a time into the sauce. Let each chunk melt before adding the next. The sauce will thicken up quite a bit during this step, which should take about 5 or 6 minutes. Once all the cheese has been added, add a pinch of salt and a few dashes of white pepper. Taste, then add more seasoning as needed. Stir in the spinach.

Turn the pot down to minimum while you cook the macaroni in a large pot of salted water. Drain the pasta well, then add it to the sauce. Stir to coat. Let stand a few minutes to set, then serve with a sprinkle of the crispy bacon on top.