Heat the oil and butter in a large frypan or skillet over a medium heat.
Add the salmon and cook 3-4 minutes on each side, or until barely cooked through.
Meanwhile, Mix the cornstarch and water to a paste, set aside.
Using a small saucepan, bring the cream and the milk to the boil. Thicken with some or all of the cornstarch paste, whisking constantly. Stir in the lemon juice, finely grated zest, mustard and season with salt and pepper.
Serve the salmon drizzled with the lemon mustard sauce. Sprinkle with parsley to garnish.