Brown sausage in a skillet over medium heat, breaking it into crumbles.
Sprinkle flour over the sausage, stir to coat, and cook for 2 minutes.
Slowly add milk while stirring. Bring to a boil, then simmer until thick, about 5 minutes.
Stir in salt, pepper, sage, and red pepper flakes (if using). Remove from heat.
Preheat oven to 375°F. Line a 9-inch pie dish with one sheet of puff pastry.
Pour in the gravy. Cover with the second sheet of pastry, trim, and crimp edges.
Cut slits in the top. Bake for 25–30 minutes until golden and crisp.
Let cool for a few minutes. Slice and serve warm.