Rinse and pat dry fish with paper towels. Using a very sharp knife, thinly slice fish at an angle.
Combine fish, soy sauce, pepper, and sesame oil in a bowl. Mix well and allow it to marinade for 30 minutes.
In a large pot, rinse rice till water runs clear, about 3 to 4 times, drain.
Add smashed ginger and 10 cups (2.4 liters) water to the rice and set it on the stove to boil. When water comes to boil, remove lid and allow it to simmer on medium heat for about 30 minutes. If porridge gets too thick at any point, stir in ½ cup (120ml) water to dilute.
Add marinated fish and salt to the porridge. Stir to get everything well mixed. Let porridge come back up to a boil and allow it to cook for another 3 minutes.
Stir in fish sauce, if using. Turn off heat.
Serve in individual bowls topped with green onions, ginger, cilantro, fried shallots, and yew char koay. Drizzle with a few drops of sesame oil and soy sauce. Add a little more pepper if desired.