Cut the tuna into 1/4-inch-Wide (6 mm) strips, then into 1/4-inch (6 mm) cubes.
Transfer to a bowl, cover, and refrigerate.
Place the mustard, oil, soy sauce, vinegar, dissolved vegetable powder, salt, and pepper in a blender and combine. Refrigerate.
Before serving, peel the avocado and cut it into (13 mm) cubes. Using a fork, gently combine it with the tuna. Season to taste.
TO SERVE: Place equal amounts of sauce on four appetizer plates. Pack the tuna-avocado mixture tightly into a 1/2-cup (125 ml) measure and invert it onto the sauce.
Garnish with chives.