Heat half the oil in a large frying pan over medium-high heat. Cook salmon, skin-sides down, for 6 minutes. Turn over. Cook for 4 minutes or until salmon is just cooked through. Transfer to a plate. Rest, loosely covered with foil, for 2 minutes.
Heat remaining oil in same pan over medium-high heat. Cook onion, stirring, for 4 minutes or until softened. Add stock and cream, then bring to the boil. Reduce heat to medium and simmer, uncovered, for 3 minutes or until sauce thickens slightly.
Meanwhile, heat mash according to packet instructions.
Juice 1 lemon and cut the remaining lemon into wedges. Stir capers, chives, dill and lemon juice into sauce. Season. Return salmon to pan and spoon over a little sauce. Serve with mash, green beans and lemon wedges.