Peel the asparagus in the lower third. Heat the oil in a pan and fry the asparagus over medium heat to the desired cooking point. Sprinkle a pinch of sugar on top and toss. Then take the asparagus out of the pan and place in a shallow bowl. Clean and halve the mushrooms. Melt the butter in the pan and fry the mushrooms, take them out of the pan and arrange on the asparagus. Cut the spring onion into fine rings and finely dice the bell pepper. Spread everything over the vegetables and marinate with vinegar, oil, salt and pepper. It is best to serve it lukewarm.