Preheat oven to 450°F (232°C). Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
Cut in the cold butter with a pastry cutter or your fingertips until the mixture resembles coarse crumbs with pea-sized bits.
Pour in the cold buttermilk and gently stir just until a shaggy dough forms—do not overmix.
Turn the dough onto a lightly floured surface. Pat into a 1-inch-thick rectangle, fold in thirds like a letter, rotate, and pat out again. Repeat 1–2 times for flaky layers.
Pat to 1-inch thickness and cut biscuits with a round cutter (or slice squares with a knife). Place so they just touch on the prepared sheet and brush tops with a little buttermilk.
Bake 12–15 minutes, until the biscuits are tall and golden brown.
While biscuits bake, cook the turkey sausage in a large skillet over medium heat, breaking it up with a spoon, until browned and fully cooked.
Sprinkle flour over the sausage and stir for about 1 minute to form a roux.
Gradually whisk in the milk, scraping the pan. Bring to a gentle simmer and cook 4–5 minutes, stirring, until thick and creamy.
Season the gravy with salt, plenty of black pepper, and red pepper flakes if using. Adjust thickness with a splash of milk as needed.
Split warm biscuits and ladle turkey sausage gravy over the top. Serve immediately.