Seriously?
You want someone on the internet to tell you if it's safe to eat a mushroom in your yard!!!??

...Don't do it dude. Or: Yeah, no prob it's fine... (honestly)

Go buy a book on mushrooms. Audubon Society Guide is a good one. Narrow the choices by following the guide meant for this purpose. Do a spore print. If you can't do that, DON'T EAT IT. Mushrooms are not always easy to identify. If you have doubts, DON'T EAT IT. Very, very doubtful it is a Portabella. Go buy some from the store. If it isn't obviously one of the well known edible varieties which could grow in one's backyard, It probably isn't choice even if it is edible...

...good candidate for "life beyond".