In a large saucepan or skillet over medium-high heat, cook the pork sausage, crumbling it with a spoon, until well browned and cooked through (about 8-10 minutes). Tilt the pan and use a paper towel or spoon to carefully blot out or remove most of the excess grease, leaving about 2 tablespoons of rendered fat in the pan.
Reduce heat to medium-low. Add the butter to the pan with the sausage and reserved fat. Once melted, sprinkle the flour over the sausage. Stir constantly for 1-2 minutes until the flour is incorporated and lightly toasted – this cooks out the raw flour taste.
Gradually whisk in the half-and-half, a little at a time at first, ensuring the mixture is smooth before adding more. Continue whisking until all the half-and-half is incorporated.
Bring the gravy to a gentle simmer over medium heat, then reduce to medium-low. Cook, stirring often, for 5-10 minutes, or until the gravy has thickened to your desired consistency. Stir in the dried thyme, crushed rosemary, and red pepper flakes. Season generously with freshly ground black pepper. Taste and adjust seasonings if needed.
Split the warm biscuits in half. Ladle the hot sausage gravy generously over the biscuits and serve immediately.