Preheat the oven to 200 degrees C. Cut out four pieces of foil, each measuring about 25 × 30cm. Mix together the teriyaki marinade, oil and vinegar in a small bowl.
Place a salmon steak on each piece of foil. Top each steak with one-quarter of the capsicum slices and spoon over one-quarter of the teriyaki mixture. Bring two sides of the foil together to make loose parcels, then crimp the edges to seal so that none of the juices can escape.
Place the foil parcels on a baking tray and bake for 15–20 minutes. To test if the salmon is cooked, carefully open one of the parcels and cut into the centre of the salmon. The flesh should be pale pink and flake easily.
While the salmon is cooking, toss together the spinach, watercress and bean sprouts and divide evenly among four plates.
Top each salad with a salmon steak and capsicum strips, then drizzle the cooking juices over the top and serve at once.