Season tuna with salt and pepper on each side. Combine black and white sesame seeds and roll tuna in mixture. Set aside. In a saucepan simmer shallots, thyme, pepper, bay leaf and white wine. Reduce slowly until dry. Add heavy cream and bring to a boil. Slowly whisk in butter, one piece at a time. Season with salt and pepper, strain, and keep beurre blanc warm. In a large skillet sear tuna, lightly brushed with olive oil, for 2 minutes on each side. Slice into dozen medallions. Serve immediately on serving plates. Drizzle with beurre blanc and wasabi mixed with the rice vinegar.