Heat the water bath of an immersion circulator to 176°F.
Peel the onion while keeping the root intact. Through the root, cut the onion into 8 wedges.
Add onion to a mixing bowl, then toss with oil, salt, and togarashi.
Transfer to a vacuum bag and seal. Cook sous vide for 30 minutes. Let cool and reserve.
In a pot over medium heat, combine shoyu, mirin, sugar, and demi-glace. Reduce the mixture by 20%.
Remove from heat and let cool. Stir in citrus juices. Set aside.
Season wagyu with salt. Let sit at room temperature for 30 minutes.
Prepare and heat a Binchotan charcoal grill.
Add the wagyu and sear on each side until the internal temperature reaches 124°F.
Remove from heat and let rest for half of the time it took to cook.
Place 1 piece Togarashi Onion over the Binchotan and grill until charred and cooked through.
Return the Black Garlic Ponzu to medium flame and heat until warmed through.
Slice the seared wagyu into 4 slices, then arrange the slices on a serving plate.
Garnish each piece with fleur de sel.
Separate the grilled Togarashi Onion into 4 petals, then place 1 petal over each slice of wagyu.
Drizzle the Black Garlic Ponzu over the wagyu and Togarashi Onion.
Serve with a side of Black Garlic Ponzu for dipping.
