Cook pasta in a large pot of boiling
salted water according to package
directions. Add broccoli rabe in the
last minute of cooking; drain and
transfer to a large bowl.
Melt butter in a large skillet over
medium heat. Add sausage; sear
until browned on both sides,
8 minutes. Transfer sausage to a
cutting board, slice on the bias, and
return to skillet for 4–5 minutes to
finish cooking; transfer to a paper-towel-lined plate.
Stir in garlic and pepper flakes and cook 30 seconds. Whisk in flour and cook 1 minute.
Slowly whisk in milk and zest.
Bring sauce to a boil, whisking
constantly until thickened, 8–9
minutes; season with salt and black
pepper. Add sausage back to skillet
to heat. Toss sauce and sausage
with pasta and broccoli rabe.