In a bowl, smash avocado pulp from one avocado with the yuzu kosho, lemon juice, and kosher salt to taste.
Toast the sourdough slices on a griddle with grapeseed oil, until golden brown.
Spread the smashed avocado pulp on the pieces of toast.
Cut the remaining avocado in half, remove the pit. Using a spoon, scoop out the pulp intact. Place the pit-less side down on the cutting board. Using a small paring knife, make as many thin vertical slices into the avocado as you can.
Spread the avocado out as if it were a deck of cards, about the width of the toast. Use a knife to pick up the sliced avocado in one piece and place on top of toast.
Garnish with red onion. Sprinkle furikake to taste. Garnish with marigold leaves.