Peel the lower third of the asparagus, cut away dry ends and cook until al dente. Remove, refresh with cold water, drain well and cut into thirds.
Cut away the dark and light green parts of fresh onions. Half onions lengthwise and cut into 1/2 inch thick slices.
Cut salmon into 2-3 cm cubes, salt and pepper and drizzle with lemon juice.
Mix well vinegar with 1 tbsp. water, salt pepper. Pour in olive oil, stirring constantly.
Heat olive oil and butter in a pan. Sauté Salmon and spring onions over medium heat around briefly and remove it from the pan.
Add salmon, onions and asparagus in a bowl and mix gently with the marinade.