In a shallow bowl, add 2 tablespoons of the fish sauce, the soy sauce, the oyster sauce, the brown sugar, 2 cloves of the minced garlic, and 1 teaspoon of the black pepper and whisk to combine.
Add the steak to the marinade mixture and turn it once to completely coat it.
Allow the marinated steak to sit in the bowl in the refrigerator for 2 hours. Make sure to allow the steak to come to room temperature for about 30 minutes after marinating.
In a sauté pan over medium-high heat, add the rice and cook, while stirring constantly, until it is golden-brown in color, about 10-12 minutes.
Transfer the toasted rice to a mortar or a spice grinder and grind it into a coarse powder.
In a bowl, add the remaining fish sauce, the lime juice, the tamarind paste, the sugar, 1 tablespoon of the toasted rice powder, the remaining minced garlic, 4 of the sliced red chiles, the red chili flakes, the cilantro, the shallots, the scallions, and the remaining black pepper and whisk to combine. If there is extra toasted rice powder, reserve it for another use.
Set the sauce aside.
In a cast-iron or heavy-bottomed skillet over medium-high heat, heat a thin layer of the oil.
Add the steak to the hot oil and cook, undisturbed, until seared on the bottom, about 2 minutes.
Flip the steak and cook, undisturbed, until seared on the bottom, about 2 minutes.
Cook the steak, while flipping it every 1-2 minutes, until it reaches an internal temperature of 130 degrees F, about 2-3 minutes.
Transfer the steak to a cutting board and allow it to rest until it reaches an internal temperature of 135 degrees F for medium-rare, about 10 minutes.
Slice the steak against the grain.
Serve the steak with the sauce and garnish with the extra chile slices and the parsley.