1. Preheat the broiler; position an oven rack 4 to 5 inches from the heat source. Line a baking sheet with aluminum foil, and grease the foil.
2. Put the salmon on the prepared baking sheet, lightly brush with vegetable oil, and season with salt and pepper. Grill for about 2 minutes. (If using thinner tail pieces--which cook faster--skip this initial grilling.)
3. For the gochujang glaze: In a small bowl, stir together the gochujang, mirin, sesame seeds, sugar, spring onions and sesame oil.
4. Brush the salmon with 6 tablespoons of the glaze, then broil until cooked to the desired doneness, about another 5 minutes for medium-rare. Transfer to a platter, and top with spring onions, sesame seeds and chile threads. Serve with rice.
Notes: If you happen to get skin-on salmon, don't bother removing the skin yourself; just skip greasing the foil. Put the salmon skin-side down on the ungreased foil, and lightly brush the top with oil. When cooked, the skin should stick to the foil and the fillet can be easily lifted from it with a spatula.