Preheat the oven to 375 degrees Fahrenheit & butter a 2-quart casserole.
Cook the pasta, adding a splash of olive oil to the boiling water as you add the macaroni.
Cook at a simmer for about 4 minutes, or until a little firmer than al dente. Drain and rinse thoroughly with cold water.
Toss with another splash of olive oil in a bowl, and add the shrimp, which will begin to cook in the hot pasta. Set aside.
Add the milk and Meyer lemon zest to a saucepan and heat over a low setting until just barely simmering.
While the milk heats, add the butter to a large saucepan and melt over a medium-high heat.
Add the flour to the melted butter, stirring for a minute or two.
Pour the hot lemon/milk mixture slowly into the melted butter and flour (or "roue") whisking as you do. The mixture should thicken as you stir.
Stir in the minced rosemary, salt, and pepper. Cook for about a minute.
Add the cheese a handful at a time, reserving about a quarter cup of each type of cheese. Continue to stir as you add, until the cheese is completely melted and the sauce is very smooth in texture.
Add the macaroni and shrimp to the cheese sauce and combine thoroughly.
Add the macaroni mixture to the casserole dish.
Top with the remaining cheese you've held back.
Bake in the oven for about 30 to 35 minutes, until the top is golden brown.