Preheat the oven to 350°F and spray a 9x13 inch pan with nonstick cooking spray. Cut each biscuit into 4 equal parts and place them evenly on the bottom of the pan.
Cook the sausage and onion in a skillet, breaking apart and crumbling the meat as it cooks, until onion is soft and sausage is cooked through. Add the salt, pepper and sage and stir to combine. Remove from heat and set aside.
In a small bowl, beat the eggs. Add the melted butter and milk, stirring to combine. Pour the egg mixture over the top of the biscuits and then top with the sausage mixture. Spoon the gravy over the top of everything and top it all with shredded cheese.
Place in preheated oven and bake for 35 to 45 minutes, until the biscuits are cooked through. Watch casserole closely and cover with tinfoil if getting too browned. (I usually cover it half way through baking.)
Remove from the oven, let rest for 5 minutes and then serve!
