Finely mash the tuna with a fork in a bowl, then blend in the lemon juice, mayonnaise, and spicy mustard, setting this mixture aside.
Heat olive oil in a large skillet over medium-high heat, then cook the chopped onion until it turns soft and translucent, about 5 minutes.
Add the shredded cabbage to the skillet, stirring constantly for 2 to 3 minutes until it warms through.
Pour in 1/2 cup water, cover the skillet, and let the cabbage steam gently for 5 minutes.
Take the skillet off the heat and fold the prepared tuna mixture into the cabbage until evenly mixed.
Transfer the salad into a serving bowl and chill in the refrigerator for 4 hours to let the flavors meld.
Before serving, optionally place a Japanese soft boiled egg on top and season with salt and either black or white pepper as you prefer.