At the start of "
Breakfast in France" you'll run down the hill to a few houses with a motorcycle,
jump on the bike and spin around 180 degrees. Follow this road driving away from the main objective for roughly one minute
until you reach a dirt road bearing right. As you approach it will show the side quest and blue chevrons, follow this second road to the very end to find a hanger and a box to interact with. Upon opening the box the "Mushroom Surprise" achievement will pop.