Prepare spinach and other desired salad greens (arugula, spring onions, etc.) Arrange salad on individual plates and set aside.

Wash and slice mushrooms. Heat a fry pan, add oil, and half the butter. Add the mushrooms, stir to coat with the oil and butter, and saute until lightly cooked. Remove mushrooms from the pan with a slotted spoon, leaving any dripping in the pan, and arrange attractively on the salad plates.

Add the rest of the butter to the pan, and swirl to melt. Add vinegar, and deglaze the pan. Pour this dressing carefully over the salads.

Serve at once.