How to Make It
Combine all of the ingredients in a small saucepan and bring to a boil. Remove from heat and let cool to room temperature. Strain and set aside.
In a blender, purée the ginger with 1 tablespoon of the olive oil; add more olive oil as necessary until the ginger purée is very smooth. Transfer to a bowl and stir in the Champagne vinegar, soy sauce and 1/4 cup of the Kaffir Syrup. Set aside.
Using a very sharp knife, slice the tuna steak horizontally 1/4-inch thick. Cut the tuna slices into thin ribbons that are the width of spaghetti. In a bowl, gently toss the tuna ribbons with the olive oil, Thai chile and shallot. Lightly season with salt.
In a medium bowl, gently toss the avocados with the olive oil and season to taste with lime juice and salt. Drain the radishes and pat dry. In a bowl, toss the radishes with a few drops of Chinese chili oil and season with salt. Mound the avocado mixture in four soup bowls. Top with half of the radishes. Arrange half of the tuna on top in bundles. Top with more of the radishes, saving a few slices for garnish. Arrange the remaining fish on top in bundles. Sprinkle the remaining radishes on top. Spoon 3 tablespoons of Ginger Marinade around each salad and drizzle with a few drops of Chinese chili oil. Serve at once.
Mix any remaining Kaffir Syrup with seltzer water for a refreshing limeade. Use any remaining Ginger Marinade as base for a vinaigrette.