Place the radishes in a heatproof container or jar.
In a separate microwave-safe container, combine the water, white vinegar, sugar, and kosher salt. Microwave the mixture until boiling, about 2 minutes, and pour it over the radishes. Alternatively, heat the mixture in a pot over medium heat until boiling and proceed.
Let the radish mixture cool completely to room temperature. Use immediately or store covered in the fridge for up to 3 days.
Prepare the perilla vinaigrette
In a blender, combine extra virgin olive oil, rice vinegar, honey, perilla and salt then blend until smooth. Pause to scrape down the sides as necessary until the mixture becomes homogeneous.
Taste and adjust seasoning with more salt or rice vinegar if necessary. Use immediately in the fridge for up to 3 days.
Assembly
On a large serving plate, layer alternating types of citrus and drizzle generously with the perilla vinaigrette. Places pieces of pickled radish among the citrus and top evenly with chiffonade perilla leaves.
Open the tin of tuna ventresca and break it into generous chunks or flakes. Place pieces of tuna throughout the salad, but leave the oil in the tin.