Heat the oven to 375°F. Butter a metal 9-by-13-inch pan, quarter the biscuits and spread them apart in it. Bake for 12 minutes, until the surfaces start to firm.
Meanwhile, brown the sausage in a skillet for 8 to 10 minutes, breaking it up and cooking it to 160°F. Leave 3 tablespoons of rendered fat in the skillet and drain any excess.
Stir the flour into the sausage for 1 minute. Gradually whisk in the milk, then add pepper, salt and sage; simmer for 4 minutes until the gravy lightly coats a spoon.
Lift the partly baked biscuits onto a plate. Spread the hot gravy in the pan and arrange the biscuits on top, with their pale sides facing down.
Bake for 15 to 20 minutes, until the biscuits are golden and a center piece has dry, fluffy layers when split open. Rest for 5 minutes before spooning into bowls.