Flat mushrooms are a typical white mushroom (agaricus bisporus) picked at their most develop stage when the shroud or underside has opened completely to structure a level top. Pads are the biggest of the three white agaricus stages at somewhere around 60mm and 130mm and have the darkest gills.
The strong kind of level mushrooms is like field mushrooms. They have a thick, substantial surface.
Mushrooms are a decent wellspring of folate and B vitamins, and are likewise one of the few common wellsprings of vitamin D.