• For the beef, in a wide bowl or square pan, stir together the oil, soy sauce, oyster sauce, and pepper. Add the beef and turn to coat all sides.
• Prepare a medium-high fire in a charcoal grill or preheat a gas grill to medium on one half and medium-hot on the other half. Heat source shold be indirect. Place flank steak in center of grill. Grill about 4 minutes a side for medium rare. Remove from heat. Let meat rest about 5 minutes before slicing.
• Slice beef against the grain into 1/4-1/2 inch slices then arrange on a serving platter. Sprinkle with the ground toasted rice (optional) and garnish with the cilantro. Serve with the sauce.

Sauce

• Place tomatoes, unpeeled shallots, and garlic on a small sheet pan and broil, turning often until charred and squishy. This should take about 7-10 minutes.
• While the beef rests, finish the sauce. Using a mortar and pestle or bowl and a fork, smush the shallots and garlic into a rough paste. Add the tomatoes, blend to form a chunky sauce. Don't puree the tomatoes. Transfer to a serving bowl and stir in the fish sauce, lime juice, pepper flakes, sugar, and chopped cilantro. The sauce should have the consistency of a chunky salsa. Let it sit a few minutes, then taste; add more lime juice, fish sauce, and pepper flakes, if needed. The sauce should taste primarily sour and spicy and then salty with, with a hint of sweetness.
• You could make this in a food processor, just don't overprocess the sauce! Use the pulse button only to avoid this.

Toasted Rice Powder

• Place raw glutinous rice in a small skillet over medium-low heat. Swirl pan continuously until rice is golden brown and has a nutty aroma. This will take about 10-15 minutes. Remove from heat and pour into a mortar or small bowl. Set it aside to cool. Once cooled, grind in the mortar or in a small food processor, until rice becomes a powder.
• Sprinkle this on your meat right before serving.