This was 12 minutes north of Crater Lake on Alaska Airlines from LA to Seattle almost 4pm PDT. Actually, there were two of these clouds side by side.
The dark spots are dirt on the plane window. I'm looking west.
This was the first one (southern) that we came to.
I couldn't find much about mushroom shaped clouds on line other than nuclear clouds. There was an article in The Guardian that discussed non-nuclear mushroom shaped clouds.
However, mushroom clouds are not unique to atomic explosions. Any sufficiently powerful source of heat, such as a volcano or forest fire can produce one. The heat creates a powerful updraught, channelling dust and smoke from the ground into a narrow chimney, forming the stalk of the mushroom. This chimney continues to rise until it meets an obstruction in the form of a boundary layer in the atmosphere. The rising column then spreads out and forms the cap of the mushroom.National Atlas has a map of potentially active volcanoes in Oregon, but I don't know that any created these.
Wikipedia has a picture of a cumulusnimbus cloud which it calls an anvil cloud. The stalk is a lot narrower than the two above.
Wayne Flann Avalanche blog has a picture of a mushroom shaped cumulus cloud.
I expect that these two clouds would not have been as obvious, or even visible, from the ground.
BTW, here's a picture of Crater Lake which we passed 12 minutes before the clouds above.