Preheat oven to 400 degrees F. Bake salmon at 400 degrees F for 20 minutes. Let cool and then finely chop salmon fillet.
Finely chop onion and red bell pepper. Heat oil in a saute pan and cook onion and bell pepper over medium-high heat until onions are translucent.
Combine salmon, egg whites, capers, breadcrumbs, mustard, lemon juice, onion and red pepper mixture, salt and pepper in a bowl; mix well.
Shape into eight 3-inch-round patties. Heat oil in a large nonstick skillet over medium heat. Cook salmon cakes until bottom is golden, about 6 minutes; flip and cook until golden, 5 minutes more.
Toss greens with your favorite dressing; divide salad among 4 plates. Top each with 2 salmon cakes (or if preferred you can serve them on the side).