In a non-stick skillet cook the sausage until brown, crumbling it as it cooks. Add the butter and finely chopped onion and cook on medium low heat until the onion is translucent. Sprinkle the flour over the mixture in the skillet and cook for 1 minute to remove the flour taste. Add the sage, salt, and pepper, and crushed red pepper, stirring to combine. SLOWLY pour the milk into the skillet, whisking well as you pour. If you like creamier gravy add the half-and-half, if not, using whole milk is perfectly fine. Stir until the gravy begins to simmer but do not let it come to a complete boil. Continue whacking until the desired thickness is achieved, realizing it thickens more as it cools. If it gets too thick, just add more milk. When thick, remove from heat and immediately spoon the gravy over hot, buttered biscuits.