Add the egg yolks, wasabi paste, vinegar, ginger and salt to a blender and process until smooth. With the motor running, slowly drizzle in the cooled butter. Blend until fully incorporated and smooth. Transfer to a dish or pitcher and refrigerate until needed.
For the Soy-Sesame Dressing:
Add all of the ingredients to a small, lidded jar and shake well to combine.
For the Salad:
To a large salad bowl, add the greens, cucumber, onion and radishes. Toss with a portion of the salad dressing. Arrange the dressed greens on 4 salad plates and add the avocado slice. Divide the tuna equally and arrange on the salad plate. Spoon or pour a portion of the reserved wasabi sauce to each salad. Serve immediately
Notes
Both the wasabi emulsion and the soy-sesame dressing can be made in advance and refrigerated.
Either a mini-food processor or an stick blender works well for the wasabi sauce. If using a stick blender place the ingredients inside a pint-size Mason jar and drizzle in the butter while blending.
The wasabi sauce will keep for 3 or 4 days in a covered container in the refrigerator.
Partially freezing the tuna will make slicing the tuna thinly much easier.
The wasabi emulsion will make 2 cups with 40 calories per tablespoon
The soy-sesame dressing will make 1 cup with 66 calories per tablespoon