Making 3/4 cup Essential Sweet-and-Spicy Ancho Seasoning Paste: Roast the unpeeled garlic directly on an ungreased griddle or heavy skillet over medium heat, turning occasionally, until soft (they will blacken in spots), about 15 minutes; cool and peel. While the garlic is roasting, toast the chiles on another side of the griddle or skillet: 1 or 2 at a time, open them flat and press down firmly on the hot surface with a spatula; in a few seconds, when they crackle, even send up a wisp of smoke, flip them and press down to toast the other side. In a small bowl, cover the chiles with hot water and let rehydrate 30 minutes, stirring frequently to ensure even soaking. Drain and discard the water.
Combine the oregano, black pepper, cumin and cloves in a food processor or blender, along with the chiles, garlic and 1/2-cup of the broth. Blend to a smooth puree, scraping and stirring every few seconds. (If the mixture won't go through the blender blades, add a little more liquid.) Press through a medium-mesh strainer into a bowl. Taste and season with salt, usually about a generous 3/4 teaspoon (it should taste a little salty).
Marinating the fish: Heat 1 tablespoon of the oil in a small (1- to 1 1/2-quart) saucepan over medium-high. When hot enough to make a drop of the puree really sizzle, add it all at once, and cook, stirring constantly, until very thick, about 5 minutes. Stir in the vinegar and sugar and cool to tepid. With sharp scissors, trim off all fins and cut out gills from both fish, then make 3 diagonal slashes on each side of the fish, penetrating to the bone. Place the fish in an oiled 13 x 9-inch baking dish, then smear all but 2 tablespoons of the chile paste (reserve the remainder) over both sides of each fish. Cover and refrigerate an hour or two (but not more than 6).
Baking the fish: Turn on the oven to 400 degrees. With a brush, daub 1 tablespoon of the remaining oil over the fish. Bake uncovered until the flesh comes away from the bone near the top of the head, 12 to 14 minutes.
Finishing and serving the dish: While the fish is baking, heat the remaining 1 tablespoon oil in a medium-size (8- to 9-inch) skillet over medium-high. Add the onion rings and stir-fry until browned but still crunchy, about 5 minutes. Add the reserved 2 tablespoons chile mixture and the remaining 1/2 cup of the broth to the onions. Boil quickly until reduced to a light saucy consistency. Taste and season with salt if necessary; keep warm.
Use 2 metal spatulas to transfer the fish to a large, warm, serving platter. Add any baking dish juices to the sauce, return to a boil, then spoon over the fish. Sprinkle on the chopped cilantro and radishes. Garnish with sprigs of cilantro, and carry confidently to the table.