In a mixing bowl, whisk together the flour, baking powder, salt, and baking soda.
Add the cold, cubed butter to the flour mixture. Work the butter into the flour until it resembles coarse crumbs.
Pour in the buttermilk gradually, stirring until just combined. Be careful not to overmix.
Turn the dough out onto a floured surface and gently knead it 3-4 times until it holds together. Pat it into a rectangle about 1-inch thick.
Use a biscuit cutter or a glass to cut out rounds, placing them on a baking sheet lined with parchment paper.
Bake in the preheated oven for about 12-15 minutes, or until the tops are golden brown. Remove from the oven and set aside.
In a medium saucepan over medium heat, melt the unsalted butter. Add the chopped onion and garlic, sautéing until the onion is translucent.
Stir in the flour, cooking for 1-2 minutes to form a roux.
Gradually whisk in the turkey broth, ensuring there are no lumps. Bring the mixture to a simmer and let it thicken, stirring occasionally.
Once the gravy has thickened, add the cooked turkey and season with salt, pepper, and thyme or sage if using. Cook for an additional 2-3 minutes until heated through.
Split the warm biscuits in half and generously drizzle or spoon the turkey gravy over the tops.
Notes
Be careful not to overmix the biscuit dough for a flaky texture.