In a blender or food processor, combine the strawberries, yuzu juice, lemongrass, sugar, and water. Blend until completely smooth. Taste and adjust seasoning or sweetness, if needed. Chill in the refrigerator for at least 30 minutes.
In a small bowl, gently toss the avocado, cucumber, strawberries, and kaffir lime leaf. Drizzle with lemon juice and olive oil, then season with salt. Mix until well combined.
Lay the tuna slices on a serving plate. Spoon a small amount of the salad onto each slice, rolling or folding the tuna to hold the filling, if desired.
Just before serving, pour the chilled strawberry soup around the tuna. Garnish with extra herbs, a sprinkle of flaky salt, or a drizzle of olive oil, if you like. Serve immediately.