Rinse mushrooms and drain well. Heat oil in a pan and fry mushrooms for about 3 minutes. Season with salt and pepper.
Mix together vinegar, salt, pepper and sugar. Add mushrooms and let cool.
Rinse scallions and cut into thin rings. Chop celery. Rinse, shake dry and finely chop dill. Mix everything with the marinated mushrooms. Let soak again for about 10 minutes. Season with salt and pepper. Serve with hearty rye bread, as desired.