In a small pot, add 1/4 cup olive oil and the medium diced shallot. Heat over low for 30-45 minutes until the shallots are very soft. Be sure to keep the heat low to avoid any browning. When finished, remove from the heat and cool to room temperature.
Reserve 1-2 tbsp of the shallot oil for toasting the bread.
In a large mixing bowl, add the cooled shallots with the remaining olive oil it was cooked in, along with 1 tbsp white soy sauce and 1 tbsp yuzu ponzu sauce. Whisk until emulsified as a dressing.To the mixing bowl, add 4-6 oz diced tuna belly pieces. Gently toss to combine with the dressing until fully coated.
Toast a slice of sourdough bread in a pan using the shallot olive oil until it is golden on both sides. (You may need to add more olive oil, depending on how much your bread absorbs)
Slice the sourdough in half, but keep the halves together and place on your serving plate. Use a slotted spoon to scoop the tuna and shallots onto the toasted bread. Garnish with a pinch of black sesame seeds, and thinly sliced green onions.