Preheat your oven to 350°F. Grease a 9x13-inch baking dish.
In a large bowl, combine the biscuit mix and 2/3 cup of milk. Stir until just combined. Spread the mixture evenly in the bottom of the prepared baking dish.
In a skillet over medium heat, cook the breakfast sausage and chopped onion until the sausage is browned and cooked through, about 5-7 minutes. Drain excess fat if necessary.
Sprinkle the flour over the cooked sausage and onion, stirring to coat. Gradually add 2 cups of milk, stirring constantly until the mixture thickens, about 3-5 minutes. Add garlic powder, onion powder, black pepper, and salt. Mix well.
Pour the sausage gravy over the biscuit layer in the baking dish. In a small bowl, whisk together the eggs and pour them evenly over the casserole.
Sprinkle the shredded cheddar cheese on top. Bake in the preheated oven for 30-35 minutes, or until the casserole is set and the cheese is bubbly.
Let it cool for a few minutes before slicing and serving.