Bring a large pot of well-salted water to a rolling boil and add your pasta. Cook it until just al dente, following the package directions as a guide. Before draining, scoop out about a cup of the starchy pasta water and set it aside for later.
While the pasta cooks, melt the butter in a large skillet over medium heat. Add the minced garlic and cook for about a minute, stirring often so it doesn't burn. You want it fragrant and golden, not brown or bitter.
Pour in the white wine, if using, and let it bubble for a minute to cook off the sharp alcohol taste. Stir in the heavy cream, lemon juice, and lemon zest, then let the mixture simmer gently so the flavors start to meld together.
Sprinkle the flour over the sauce a little at a time, whisking constantly so no lumps form. Let it simmer for a few minutes until you notice it clinging nicely to the back of a spoon. This is also when you'll stir in the parmesan cheese.
Add the shrimp directly into the simmering sauce and cook for three to five minutes, just until they turn pink and opaque. Avoid overcooking, since shrimp can go from tender to rubbery in a matter of seconds.
Drain the pasta and add it straight into the skillet with the sauce. Toss everything together, using a splash of the reserved pasta water if the sauce needs loosening. Finish with chopped parsley and serve immediately while hot.