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, Calories
4.5, Fat Calories
40, Saturated Fat
6, Monounsaturated Fat
8, Polyunsaturated Fat
Photo: Scott Phillips