On medium-high heat, heat oil in a medium skillet (or cataplana if you have it!) and add onions. Cook until slightly translucent, stirring about 3 minutes. Add garlic and mushrooms and reduce heat to medium. Cook 5 minutes, then add wine and tomato. Cook 3 minutes more until liquid is reduced. Add salt and parsley and adjust taste if necessary. You may simmer longer to intensify the flavors in the mushrooms.