Cut the fillet into steaks. Tenderize it carefully with a wooden mallet, without hitting hard.
Peel the mushrooms and onions and cut them into suitable pieces. Season with salt and black pepper.
Arrange the steaks in an oven dish with a nonstick bottom. Add the mushrooms and onions. Pour in the white wine and sprinkle with olive oil.
Bake in a preheated oven until golden but don't bake too much so they don't end up dry.