Heat oven to 425°F and line a sheet pan with parchment.
Toss broccoli with sesame oil, salt, garlic powder, onion powder, and pepper until well coated, then spread in an even layer on the pan.
Roast broccoli 8–10 minutes to get it started.
Meanwhile, whisk together gochujang, soy sauce, sesame oil, ginger, onion powder, garlic powder, black pepper, and salt.
Pull out the pan, push broccoli to the sides, place cod in the center, and brush generously with the sesame gochujang sauce.
Roast 10–12 minutes more, until cod flakes easily and broccoli is tender with some char on the edges.
Garnish with sesame seeds and green onion, squeeze over citrus (lime or lemon), and serve.