Cook sausage in a skillet over medium heat until browned, breaking it into crumbles.
Sprinkle with flour, stir to coat, and cook for 2 minutes.
Gradually add milk, stirring constantly. Bring to a boil, then simmer for 5 minutes until thick.
Stir in salt, pepper, sage, and red pepper flakes (if using). Remove from heat.
Set oven to 375°F (190°C).
Roll out one puff pastry sheet into a 9-inch pie dish. Pour in the sausage gravy and spread evenly.
Place second pastry sheet over the top, trim, crimp edges, and cut slits for steam.
Bake on the center rack for 25–30 minutes until golden brown.
Cool for 5–10 minutes before slicing and serving.