Grilled Whole Sea Bream with Chile Glaze

Toast guajillo and ancho chiles in heavy large skillet over medium-high heat until slightly darker in color, 1 to 2 minutes per side for guajillo chiles and about 3 minutes per side for ancho chiles. Using tongs, carefully transfer toasted chiles to sheet of foil and cool. Remove stems and seeds from all chiles, then crumble chiles finely into small bowl. Sprinkle sugar over bottom of heavy large saucepan. Cook over medium heat without stirring, occasionally swirling and tilting pan, until sugar dissolves and becomes medium golden brown syrup, 7 to 8 minutes. Turn off heat. Add chopped garlic and stir 20 seconds. Mix in agave nectar and crumbled chiles; stir 1 minute. Add all citrus juices. Bring mixture to boil over high heat, stirring occasionally. Reduce heat to medium-low and simmer until glaze thickens enough to coat spoon, about 20 minutes. Strain glaze into small microwave-safe bowl, pressing hard on chile solids to release all liquid. Discard solids in strainer. Mix in fish sauce. Season glaze to taste with salt and pepper. DO AHEAD: Glaze can be prepared up to 3 days ahead. Cool, cover, and refrigerate. Rewarm glaze in microwave before continuing. Mix 1 tablespoon chopped fresh cilantro into glaze.