At this point you can eat as is, or you can put into a casserole, top with a little more cheese and broil it for 5-10 minutes.
1 Blanch spinach and parsley: Bring a large pot of water to a boil. Add the spinach and parsley and blanch for 1 minute. Use a slotted spoon to remove the wilted spinach and parsley to a bowl of ice water. (Do not discard the blanching water, you will use it to cook the macaroni.)
Strain the spinach and parsley. Wrap in paper towels and squeeze out excess moisture.
2 Use blanching water to cook macaroni: Return blanching water to a boil. Add salt to the pasta water, 1 Tbsp salt for every 2 quarts of water. Add the macaroni pasta and cook until very al dente. (If the box of macaroni says to cook it 7 minutes, cook it 5. It will continue to cook and absorb moisture once you mix it with the sauce.)
Drain the macaroni, rinse with cold water to stop the cooking, and set aside.
3 Purée spinach with cream: While the macaroni is cooking, in a food processor, purée the cooked spinach mixture with the tablespoon of cream until smooth. Set aside.
4 Make roux with butter, garlic, flour: Melt 3 Tbsp butter in a thick-bottomed pot on medium high heat. Add the minced garlic and cook for 20-30 seconds. The butter should bubble and foam. Add the flour and whisk to form a paste. Lower the heat to medium and cook for 1 to 2 minutes.
5 Slowly add milk: Slowly add the milk, whisking as you add it to break up any clumps of flour. Increase the heat to medium high and let the mixture come to a simmer, continue to whisk.
Lower the heat to medium low and let cook until the sauce is thick enough to coat the back of a spoon.
6 Add dry mustard, pepper, herbs, Worcestershire: Stir in the dry mustard, black pepper, herbs, and Worcestershire sauce if using.
7 Add the cheeses: Whisk in the 1 1/2 cups of grated white sharp cheddar and 1/2 cup grated Parmesan. Stir until the cheese has melted and the sauce is smooth.
8 Add puréed spinach and parsley: Stir the puréed spinach and parsley into the sauce. If using finely chopped anchovies, you can add them in now. Taste the sauce for salt. Add more if needed.
9 Add cooked macaroni: Stir in the cooked drained macaroni pasta. The pasta will continue to absorb liquid as it sits in the sauce, so don’t be alarmed if the mac cheese seems really loose (the macaroni will continue to absorb moisture as it sits). Stir in lemon juice.