So, to make them... Here ya go:
Make the burger as you like them.
Go prepare the mushrooms you have options.
If you have a grill, like a foreman or something, just use that. You'll know when they are done. Simply brush the mushroom with a light coat of olive oil, then put them on for a few minutes. You'll know, just trust me.
If you don't, then you can use a skillet.
To use a skillet:
Get another skillet slightly smaller with two 14 oz cans. Heat the larger skillet to medium. Brush outside of mushroom with olive oil. Put the mushroom in the larger skillet, then use the smaller skillet with two cans in it to weigh down the mushroom. This will help flatten out the mushroom. It only takes a few minutes.
Make the burger as you would any other burger and enjoy. One HUGE difference is the juiciness. I suggest eating the burger out of a couple of paper towels. It can be messy!