Heat oven to 425 degrees. Put flour, baking powder, sugar and salt into a food processor or into a large bowl. Pulse or stir to mix. Drop diced butter and shortening randomly over the flour mixture. Pulse or use two knives to cut the butter and shortening into the flours until the mixture resembles coarse crumbs. Stir in herbs. Sprinkle the milk over the mixture. Pulse or stir until the mixture barely gathers into a ball.
Transfer to a piece of floured wax paper; gather into a ball. Cover with a second piece of floured wax paper; gently roll or press into an even 1/2-inch thick circle. Use a floured 3-inch round biscuit cutter or drinking glass to cut into circles. Place the circles on a parchment-lined baking sheet. Very gently gather up the scraps; roll into a 1/2-inch thick piece and cut more biscuits.
Brush the tops with milk; bake until tops are golden, 12-15 minutes.
Crumble chorizo sausage into a cast-iron pan or nonstick skillet. Add onion. Cook until nicely browned, about 10 minutes. Transfer sausage and onion with a slotted spoon to a plate.
Drain off all but 2 tablespoons of the pan drippings. Mix 1/2 cup of the milk and the flour in a small bowl until smooth. Stir into pan drippings; stir in remaining milk. Cook, stirring, over medium heat until smooth and thickened, about 5 minutes.
Stir in sausage, diced red pepper, salt and black pepper to taste. Heat to warm through, 1-2 minutes. Serve over biscuits; sprinkle generously with cilantro.