Pour the oil into a small saucepan to a depth of about 1 inch.
Heat the oil until a small piece of shallot sizzles rapidly when added to the pan.
Toss the shallots with the flour. Carefully add the shallots to the oil and fry until golden brown, 30 to 60 seconds. Remove the shallots with a slotted spoon and drain on paper towels. Season with a pinch of salt.
Make the Dressing
In a medium bowl, stir together the wasabi powder and 1 tablespoon water to make a smooth paste.
Whisk in the soy sauce, mirin, mustard oil, vinegar, and honey.
Add black pepper to taste. Set aside.
Make the Tartare
Toss the tuna with the olive oil, salt, and pepper to taste.
Season the avocados with salt and pepper to taste.
To Serve
To mold the tuna into a disk, place a 3-inch tartlet ring or an empty tuna can with the top and bottom lids cut out on a chilled plate. Fill the ring with a layer of avocado and top with the diced tuna and fried shallots. Lift off the ring. Drizzle the sauce around the tuna.