Cut the onion into half rings.
Mix the vinegar, oil, and mustard into a homogeneous emulsion.
Add a little salt to the onion, mash it with your hands and pour the dressing over it.Wash the potatoes thoroughly, put them in a saucepan, cover with water and cook until tender for 20-25 minutes.
Peel the boiled potatoes and cut them into half-circles.
Cut the sausage into thick strips. Spread on a dish with potatoes.
Spread the fried mushrooms. I have a winter preparation, the last jar. If there are no ready-made ones, then you will have to fry them.
Cut the tomatoes into large pieces and add them together with the onion and liquid to the salad. Mix everything well. Season with salt and pepper.
Leave the salad for 20-30 minutes to soak.
Sprinkle the salad with green onions when serving.