Pat the salmon dry with paper towels and lightly salt and pepper both sides. Place the salmon in a zipper-lock plastic bag.
In a small bowl, whisk together the hoisin sauce, ginger, soy sauce, vegetable oil, half of the scallions and the crushed red pepper flakes. Reserve 2 tablespoons of the marinade to use as the glaze when cooking the salmon. Pour the remaining hoisin glaze/marinade over the salmon in the bag, seal the bag, pressing out as much air as possible. Refrigerate at least 30 minutes but no longer than 10 hours. While the salmon marinates, flip the bag over 3-4 times to ensure it marinates evenly.
Refrigerate the reserved glaze in a small airtight container until ready to use.
Wash and dry the asparagus. Cut off the woody ends and peel any thick spears from about halfway up toward the root end. Do NOT peel the tips and be careful not to peel too much of the core, which is sweet. Wrap in a paper towel and refrigerate until ready to use. This asparagus doesn't need to be peeled.
Remove the salmon and asparagus from the fridge 15 minutes before cooking. Set the oven rack to the middle position, and heat the oven to broil. Cover a rimmed baking sheet with foil and spray the foil with nonstick cooking spray.
Transfer the salmon to the middle of the prepared baking sheet and lay it skin-side down about 1-inch apart. Place half of the asparagus on each side of the salmon and lightly salt and pepper the asparagus. Drizzle the salmon and asparagus with a little olive oil, and spoon 1-tablespoon marinade over each salmon fillet. Drizzle the salmon and asparagus with the juice from ½ lemon. Discard the extra marinade from the bag.
Broil the salmon & asparagus 7 minutes, remove from the oven and drizzle the salmon with the 2-tablespoons reserved glaze/marinade. Flip the asparagus over.
Broil the salmon and asparagus another 2-3 minutes or until the center of the salmon is still translucent and the internal temperature reads 125°F on a meat thermometer. The asparagus should be al dente.
Drizzle the salmon and asparagus with the juice from the remaining half lemon, sprinkle with chopped green onions and/or cilantro. Serve & enjoy!