Preheat the oven to 180C/350F/Gas 4.
For the roasted salmon, rub the olive oil into the salmon and season well with salt and freshly ground black pepper.
Heat an ovenproof frying pan over a high heat. Add the salmon, skin-side down first, and fry for 1-2 minutes, or until the skin is crisp. Turn the salmon over and fry for 30 seconds to one minute to sear the fish. Turn it over again so it is laying skin-side down and transfer the pan to the oven for 7-8 minutes to roast the salmon.
For the sweet and sour peppers, heat the oil in a separate non-reactive frying pan over a medium heat. Add the pepper pieces and fry for 2-3 minutes, or until they start to soften.
Add the red wine vinegar and sugar and continue to cook for 5-6 minutes, or until the peppers are soft and the vinegar has reduced.
To serve, scatter the mixed leaves onto a serving plate, and place the crisp roasted salmon on top. Arrange the peppers around the edge of the plate. Drizzle the dish with balsamic vinegar.