Slice the onion thinly and divide the broccoli into small florets.

Boil water in a pan, add salt, and cook macaroni according to package instructions. Add broccoli 2 minutes before macaroni finishes cooking, then drain.

Wipe the pan, add half the butter, onion, and shrimp. Sauté over medium heat until the onion softens, then set aside.

In the same pan, melt the remaining butter, add flour, and stir until smooth. Gradually add milk while stirring over low heat until thickened. Add macaroni, broccoli, shrimp, and onion, mix well, and season with salt and pepper.