Combine white wine vinegar, white wine, and shallots in a saucepan over medium heat. Simmer until liquid is reduced to about 2 tablespoons. Strain out shallot and discard, return liquid to pan.
Stir wasabi and soy sauce into reduction in pan. Over low heat, whisk in butter one cube at a time allowing mixture to emulsify. Do not to let the mixture boil.
When all butter has been incorporated, stir in cilantro, and remove from heat. Pour into a small bowl, and set aside.
Heat a large skillet (iron skillet works great for this) over medium-high heat. Brush tuna steaks with olive oil, season with salt and pepper. Place in hot skillet, and sear for 3 to 5 minutes on each side. .
Do not to overcook, fish should have a little pink in the center. Serve with sauce
Lacking shallots, you can substitute the mildest onion available. Lacking soy sauce, worcestershire works well.