Add the pork to a dutch oven over medium heat. Cook the pork until browned, breaking up the pork with a wooden spoon or spatula while it cooks, about 10 minutes.
Add the butter, sage, garlic, crushed red pepper, 1/2 tsp salt, and 1/4 tsp pepper and stir until well combined and butter has melted.
Add the flour and stir until thoroughly combined. Slowly pour in the milk as you stir. Increase the heat to medium-high and continue to stir until it just begins to boil.
Reduce heat to low and simmer, stirring occasionally while scraping the bottom of the pan with a wooden spoon, until thickened, 5 to 10 minutes. Season with salt & pepper to taste.