How to Make It
Preheat the oven to 325°. Spread a 1/2 inch layer of kosher salt in a pie plate. Set the shallots in the salt, root end down. Bake for 1 hour, or until very tender. Let cool slightly, then peel the shallots.
In a small bowl, mix the wasabi powder with the hot water. Cover with a plate and let stand for 5 minutes. Scrape the wasabi into a mini processor, add the roasted shallots, lemon juice, soy sauce, vegetable oil and sesame oil and puree. Season with kosher salt.
In a bowl, mix the tuna with the mayonnaise and sambal; season with salt.
Set a large skillet over moderately high heat. Add the spinach, a handful at a time, and cook, stirring, until just wilted. Transfer the spinach to a colander and let cool, then squeeze out any excess liquid. Coarsely chop the spinach and transfer it to a bowl.
In a small skillet, toast the sesame seeds over moderately high heat, stirring, until golden brown, about 1 minute. Transfer to a mortar and let cool completely, then grind coarsely. Stir the sesame seeds into the spinach along with the scallion and sesame oil and season with salt.
Stack 4 sheets of the phyllo dough on top of each other on a work surface. Brush the top sheet with melted butter and cut thr stacked phyllo into 6 squares; work with 1 square at a time and keep the rest covered with plastic wrap. Spoon about 3 tablespoons of the tuna and 1 tablespoon of the spinach on the lower third of the phyllo square. Fold the phyllo up and over the filling and roll up tightly, pushing the phyllo in at the sides to form an even cylinder. Brush the seal with butter. Transfer to a baking sheet and refrigerate uncovered. Repeat with the remaining phyllo, butter and filling to make a total of 12 rolls.
In a medium skillet, heat 1/4 inch of vegetable oil until shimmering. Add 2 of the tuna rolls and fry over high heat, turning to brown all 4 sides, about 2 minutes. Transfer the rolls to a cutting board. Repeat the process with the remaining rolls, adding more vegetable oil if necessary and reducing the heat if the rolls brown too quickly. Slice each roll into 4 pieces and arrange on a platter. Spoon 1/2 teaspoon of the shallot-wasabi sauce on top of each slice and serve immediately.
If possible, use homemade mayonnaise or a sweet Japanese brand, such as Kewpie, which is available at Asian markets.