Add the onion and cook until all vegetables are translucent.
Push the vegetables to the side of the pan.
Place the butter in the center.
Add the egg.
Flip the egg once it is stable.
Test for the desired doneness by pressing on the yolk to feel it. If it is very soft to the touch, you still have a very runny egg, this is not recommended. If your egg has a little firmness but still soft like a marshmallow you have a medium egg. That is how I did mine. If the egg has no softness and is firm, you have a hard egg.
Add the cheese slice to the top then add the vegetables.