Mix mayonnaise, liquid seasoning, oyster sauce, and calamansi juice in a small bowl. Taste and adjust with salt and pepper if desired. Cover and refrigerate until ready to use.
In a pan over medium-high heat, add the drained tuna and cook until the tuna is dry. At this point, you can either add the sisig sauce or cook the tuna a few more minutes to brown it.
When you reach the preferred texture of your tuna, transfer to a plate.
Using the same pan over medium heat, add some of the reserved oil from the tuna. Add the chopped onion and chilies, and cook until softened.
Add the sisig sauce and cook for 1 minute. Return the browned tuna to the pan and mix well.
Turn off the heat and transfer to a serving plate. Optional: garnish with chopped onions and additional chili.