Thanks to a friend who helped research what this is, we now know this is an Agaricus augustus Fries or common name "the Prince" mushroom. Actually more common to California than the PNW, apparently it can show up here as is evident. They like foggy warmer weather, which we had a spell of recently. The final cap size is evident in the photo just below.
I said "final" because the darn things are considered one of the best edible mushrooms there are, and reportedly somewhat rare. They are reported to have the slight flavor/scent of almonds, which indeed they do! You can see the bottom side below, just after we harvested it.
Given it was at a fairly mature state, we chose to slice it and dehydrate it for future use.
Obviously a note of caution for foragers in that you want to be abslolutely sure the variety you are harvesting is the edible type you think it is. Many varieties of mushroom can be quite deadly, so be absolutely sure before eating!