In a large, heavy-bottomed skillet (a cast-iron skillet works wonderfully), cook the ground breakfast sausage over medium-high heat, breaking it up with a spoon as it browns. Cook until no pink remains and the sausage is fully cooked through, about 8-10 minutes.

Once cooked, if there is more than 1/4 cup of rendered fat, carefully drain off any excess, leaving about 1/4 cup of fat in the skillet with the sausage. If your sausage is very lean and doesn't render enough fat, add 1 tablespoon of unsalted butter to the skillet now.

Reduce the heat to medium. Sprinkle the all-purpose flour evenly over the cooked sausage and rendered fat. Stir constantly for 1-2 minutes, cooking the flour to create a roux. This step helps prevent a raw flour taste.

Gradually pour in the slightly warmed whole milk, whisking constantly to prevent lumps. Start with about 1 cup, whisk until smooth, then add the remaining milk slowly, whisking continuously.

Bring the gravy to a gentle simmer over medium heat, continuing to whisk occasionally. Let it simmer, stirring often, for 5-7 minutes, or until the gravy thickens to your desired consistency. It should be thick enough to coat the back of a spoon.

Stir in the salt, black pepper, and optional red pepper flakes. Taste and adjust seasoning as needed. Remember that sausage can be salty, so add salt gradually.

Remove from heat and serve immediately over warm, freshly baked biscuits.