Heat cooking oil in a large pot over medium heat.
Add onion and garlic, stirring occasionally about 3 minutes.
After 3 minutes, add crushed ginger, lemongrass and bay-leaf, keep stirring occasionally for 1 minutes.
Add 1/2 cup of water and wait until the water boiled, then add tuna; if the tuna seasoned, you don't have to add salt anymore because it's already salty.
Add pepper and cayenne pepper; if you want to make it spicy, you can chop the cayenne pepper.
Then add another 1/2 cup of water, coconut milk and lastly green onion.
Let it cook until the sauce thickens.
Turn off the heat then put it into a bowl and garnish with green onion.
For making Seasoned Tuna:
You can substitute seasoned tuna with canned tuna.