Preparation Place racks in upper and lower thirds of oven; preheat to 300°F. Pat ribs dry with paper towels. Sprinkle evenly on all sides with mushroom powder, patting to adhere, then rub miso all over to lightly and evenly coat. Wrap each rack in a double layer of foil, crimping edges to seal. Divide foil packets between 2 rimmed baking sheets (2 packets per sheet) and bake until meat is fork-tender at the thickest part but not falling off the bones, 2–2½ hours. Let cool slightly, then unwrap. While the ribs are baking, heat oil and chiles in a medium saucepan over medium-low, stirring often, until chiles darken in color a few shades and are fragrant, about 3 minutes. Add onion and salt and continue to cook, stirring occasionally, until onion is softened and translucent, about 5 minutes. Add garlic and ginger and cook, stirring, until fragrant, about 1 minute. Add gochujang, tamari, and vinegar and cook until thickened slightly, about 5 minutes. Remove pan from heat and stir in honey. Transfer mixture to a blender and purée until smooth and slightly glossy. Pour glaze into a small bowl or airtight container and let cool. Heat broiler. Line a clean rimmed baking sheet with foil and lightly coat with nonstick spray. Cut each rack in half (this will allow you to puzzle all the ribs onto the sheet) and arrange, meaty side up, in a single layer on prepared baking sheet; brush with glaze. Broil ribs until glaze is set and slightly darkened in color (watch them closely so that the glaze doesn’t burn), about 4 minutes. Transfer ribs to a platter and top with scallions and fried shallots (if using). Do Ahead: Sauce can be made 5 days ahead; cover and chill. Ribs can be baked 2 days ahead; let cool, then wrap tightly in plastic and place in an airtight container. Cover and chill.
Preparation Place racks in upper and lower thirds of oven; preheat to 300°F. Pat ribs dry with paper towels. Sprinkle evenly on all sides with mushroom powder, patting to adhere, then rub miso all over to lightly and evenly coat. Wrap each rack in a double layer of foil, crimping edges to seal. Divide foil packets between 2 rimmed baking sheets (2 packets per sheet) and bake until meat is fork-tender at the thickest part but not falling off the bones, 2–2½ hours. Let cool slightly, then unwrap. While the ribs are baking, heat oil and chiles in a medium saucepan over medium-low, stirring often, until chiles darken in color a few shades and are fragrant, about 3 minutes. Add onion and salt and continue to cook, stirring occasionally, until onion is softened and translucent, about 5 minutes. Add garlic and ginger and cook, stirring, until fragrant, about 1 minute. Add gochujang, tamari, and vinegar and cook until thickened slightly, about 5 minutes. Remove pan from heat and stir in honey. Transfer mixture to a blender and purée until smooth and slightly glossy. Pour glaze into a small bowl or airtight container and let cool. Heat broiler. Line a clean rimmed baking sheet with foil and lightly coat with nonstick spray. Cut each rack in half (this will allow you to puzzle all the ribs onto the sheet) and arrange, meaty side up, in a single layer on prepared baking sheet; brush with glaze. Broil ribs until glaze is set and slightly darkened in color (watch them closely so that the glaze doesn’t burn), about 4 minutes. Transfer ribs to a platter and top with scallions and fried shallots (if using). Do Ahead: Sauce can be made 5 days ahead; cover and chill. Ribs can be baked 2 days ahead; let cool, then wrap tightly in plastic and place in an airtight container. Cover and chill.