Season the ahi tuna loin with salt and black pepper on each side.

Combine black and white sesame seeds in a shallow dish.

Roll the seasoned tuna in the sesame seed mixture, ensuring it's evenly coated.

Set the coated tuna aside.

In a saucepan, combine chopped shallots, fresh thyme sprig, cracked black pepper, bay leaf, and white wine.

Simmer the mixture over medium heat and reduce slowly until almost dry.

Add heavy whipping cream to the reduced mixture and bring to a boil.

Reduce the heat to low and slowly whisk in cold butter, one piece at a time, until the sauce is emulsified and smooth.

Season the beurre blanc sauce with salt and pepper to taste.

Strain the sauce through a fine-mesh sieve to remove solids and keep the beurre blanc warm.

Lightly brush a large skillet with olive oil.

Sear the sesame-crusted tuna in the hot skillet for 2 minutes on each side, ensuring a rare center.

Remove the tuna from the skillet and slice into 12 medallions.

Serve the sliced tuna medallions immediately on serving plates.

Drizzle the warm beurre blanc sauce over the tuna medallions.

Mix wasabi powder with rice vinegar to form a paste.

Drizzle the wasabi mixture over the tuna and sauce.