Cook your pasta according to the directions on the package, but undercook it by just a couple of minutes. Drain and set aside.
In the same pot that was used to cook the pasta, melt the butter over medium heat. Add the flour and whisk together. Cook the roux for about 30 seconds.
While whisking constantly, slowly pour the milk in. Whisk occasionally until the white sauce starts to thicken up. Add the citrus zest, rosemary, salt, and pepper. Whisk to combine. Add the cheddar and whisk until it is melted into the sauce. Taste and adjust your seasonings as needed.
Add the pasta to the cheese sauce and stir to coat everything. Pour it into the prepared baking dish and sprinkle the top with the shredded parmesan. Bake for 30-35 minutes, or until everything is bubbly and lightly browned. Let it cool for a few minutes before digging in.