Preheat oven to 375 degrees. Grease 13x9 baking dish. Set aside.
In a large skillet or braising dish, add diced onion and ground sausage. Cook on medium heat until sausage is no longer pink and onions are softened, about 6 minutes. Remove from skillet and set aside.
Add 4 tablespoons of butter to skillet and reduce heat to low. Once butter has melted completely, add flour and whisk together. Cook for 1 minute. Slowly add in milk, about 1/2 cup at a time and whisking after each addition to remove lumps. Once all the milk has been added, raise temperature to medium.
Cook gravy until it just begins to thicken, about 3 minutes. Add sausage and onions back to skillet with gravy. Toss together. Season with salt and pepper. Pour into prepared baking dish.
Arrange frozen biscuits on top of the sausage gravy mixture in a 4 x 3 pattern. Place dish into preheated oven and bake until biscuits have browned on top, about 35-40 minutes.
Remove from oven. Melt remaining 2 tablespoons of butter. Brush melted butter on top of finished biscuits. Let cool for 5 minutes before serving.