Heat the neutral oil in a small pot over medium-low heat. Add the onion and cook until the onion begins to sweat and soften. Do not brown the onion. It should stay very white, so adjust the heat as necessary to prevent the onion from browning.
Add the garlic and cook for 45 seconds until fragrant. Melt the butter, if using, into the aromatics. Add the wasabi oil, starting with about 1-2 teaspoons to start.
Simmer the Wasabi Broth:
Pour in the water and bring to a boil. Add another teaspoon or two of wasabi oil if desired and season the broth with salt to taste. Add the sliced daikon radish. Reduce heat and simmer for about 20 minutes.
Prepare the Eggs:
Separate the egg yolks from the whites. Discard the whites and transfer the egg yolks to a bowl. Set aside.
Prepare the Tuna:
Pat the tuna dry and rub with the sesame oil. Season all over with salt and pepper and sprinkle both sides with sesame oil. Using the palm of your hand, gently press the sesame seeds into the tuna. Transfer to the refrigerator until needed.
Finish the Wasabi Pea Broth and Eggs:
Using a spoon, ladle a few spoonfuls of the broth over the egg yolks and allow them to cook in the broth for 3-4 minutes. You do not need to completely immerse the egg yolks; just enough to warm them up. You may wish to cover the bowl with foil. If you need to, you can add a little more broth over the eggs to cook them further, but the yolk should be runny.
Add the pea shoots to the wasabi pea broth and season again to taste. Turn off the heat.
Cook the Tuna:
Heat the neutral oil in a skillet over high heat. Add the tuna and cook without moving for about 1 minute, depending on the thickness of the fish. Flip and cook an additional 1 minute. Remove from the heat and slice thinly.
To Plate:
Ladle the wasabi pea broth into bowls and arrange the sliced tuna on top. Carefully place a poached egg yolk next to the tuna. Garnish with flaky sea salt and a drizzle of mustard or wasabi oil, if desired. Enjoy!