Purchase mushroom from your local supermarket or grocery shop. Rinse and clean them thoroughly in cold water. It is recommended to cut off the stems as well and then place them in a bowl or a dish.
Consider applying pesto on both sides of the mushroom. This can be done by using a table spoon or a basting brush. Next, put some non stick cooking spray or vegetable or sun flower oil in the frying pan.
Set your stove at about 120 degree centigrade and heat the pan for about 5 to ten minutes. Toast both sides of each bun once the pan is heated to the required temperature. You may also choose to toast two to three buns at once depending on the size of your frying pan.
Flip the buns frequently to ensure they do not burn. Remove buns from the pan when they are toasted to your satisfaction. Place them open faced on a dish or a tray and allow to cool for 3 to 4 minutes.
Now add mushrooms on the same frying pan. Use a spatula to make sure they are cooked properly. Next, cut small slices of onions and tomatoes and place them on bottom half of the buns. Remove the mushrooms from the pan when they become firm.
Put a slice of cheese on each mushroom and place the mushrooms on the bottom half of the bun. Sprinkle a small amount of feta cheese on mushrooms for enhanced taste.