Wasabi powder and fresh shiso (also called perilla and Japanese basil) are available at Asian food markets. The wasabi needs to be made at least two days ahead so the bitterness fades; it keeps for up to a month. The salsa will hold for a couple of days in the fridge. The tostadas are fried wonton wrappers; egg roll wrappers also work. The tostadas will stay crisp for one week if stored in an airtight container. They make great munchies, so you might want to fry extra.
To make the wasabi paste
At least two days before serving, mix the wasabi powder with the water to form a soft paste. Cover and refrigerate.
To make the wonton tostadas
In a deep skillet, add oil to a depth of 1/2 inch. Heat the oil to 380°F -- a rice noodle will puff into a curlicue within 3 seconds or a cube of bread will turn golden in 15 seconds. (If the oil isn't hot enough, the tostados absorb too much oil and get soggy and greasy after just a few hours.) Fry a few of the wonton squares at a time until they're crisp and evenly golden on both sides, turning them during cooking if necessary, about 5 to 10 seconds. Lift out with tongs and drain on paper towels.
To make the salsa
In a bowl, stir together the tomatoes, shiso or cilantro, scallions, garlic, and rice-wine vinegar. Season with salt and pepper.
To make the tuna
In a bowl, combine the soy sauce, sesame oil, ginger, and pepper. Marinate the tuna in this sauce for 15 minutes or up to 2 hours, turning it halfway through. Heat a cast-iron skillet over high heat with 1 Tbs. oil. When the oil is very hot -- it will be smoking -- put the tuna logs in the pan and sear, 20 to 30 seconds on each side; they should be seared outside and rare inside. Slice the logs into 1/4-inch pieces.
Put a piece of tuna on a wonton. Spread a touch of wasabi paste on the tuna (not too much; it's very hot). Top with a spoonful of salsa. Garnish with a bit of crème fraîche and scallions.
nutrition information (per serving):
per tostado with 2 Tbs. salsa;
Photo: Scott Phillips