Make biscuits: Preheat oven to 450°. In a large bowl, whisk together self-rising flour, salt, and sugar. Cut cold butter into small cubes and, using a pastry blender, cut butter into flour mixture until pieces are the size of peas. Stir in buttermilk and half-and-half just until incorporated.
Put all-purpose flour in a shallow bowl or pie dish. Using an ice cream scoop or measuring cup, scoop 1/8 of dough and drop it into flour, tossing lightly to coat and shaking off excess. Put dough in an 8-in. cake pan. Repeat with remaining dough (biscuits will touch in pan). Reserve 1/4 cup remaining all-purpose flour.
Bake biscuits until deep golden brown and a toothpick inserted in center comes out clean, 20 to 25 minutes. Let cool 10 minutes. Brush with 2 tbsp. melted butter.
Make gravy: Put sausage in a medium saucepan over medium heat. Cook until lightly browned, 5 to 6 minutes, breaking it into small pieces with a wooden spoon. Transfer sausage to a plate with a slotted spoon, reserving any drippings.
Meanwhile, in another medium saucepan over medium-low heat, warm half-and-half, milk, powdered garlic and onion, pepper, thyme, cayenne, and 1/2 tsp. salt.
Add butter to sausage drippings and let melt. Add 1/4 cup reserved flour and whisk until golden, 1 to 2 minutes. Gradually add warm milk mixture, whisking, until simmering. Stir in sausage and season to taste with salt. (For thicker gravy, simmer longer; for thinner gravy, add more milk.)
Halve biscuits and put on 8 plates. Top with gravy; sprinkle with thyme.
*You can substitute 2 cups all-purpose flour mixed with 3 tsp. baking powder and 1 tsp. kosher salt.
Note: Nutritional analysis is per biscuit made with half-and-half and 1/2 cup gravy.
PER BISCUIT MADE WITH HEAVY CREAM AND 1/2 CUP GRAVY: 728 Cal., 70% (513 Cal.) from fat; 13 g protein; 57 g fat (31 g sat.); 41 g carbo (2 g fiber); 1,031 mg sodium; 163 mg chol.