Preheat oven to 450°F (230°C). Line a baking sheet with parchment paper.
In a large mixing bowl, whisk together the flour, baking powder and salt. Using your fingertips or a pastry cutter, work in butter until mixture resembles coarse meal (some larger pieces of butter are okay). Add milk gradually while stirring with a wooden spoon.
Drop heaping tablespoons full onto the prepared baking sheet and slightly spread apart to make biscuits. Bake for 12-15 minutes or until golden brown at top edges (do not overbrown bottoms, they will continue cooking in gravy). Remove from oven.
Meanwhile, heat butter with minced garlic and onion slices in a saucepan. Once fragrant add flour to the pan and stir until smoothened out (about 2 minutes over medium-high heat) removing it immediately if you notice any burnt smell.
Slowly whisk milk into roux, increasing speed while incorporating as much of mixture from saucepan. Cook gravy at a gentle simmer for about 15 minutes or until thickened (if needed add more flour).
Serve biscuits with the hot and delicious sausage gravy.