Place the pork in a bowl and sprinkle with the 2 tablespoons of soy sauce and 1 tablespoon of the cornstarch. Mix well, then let rest for 10-15 minutes.
Heat 2 tablespoons of oil in a heavy skillet or wok. Add the pork, stirring and breaking it up into small chunks while cooking. When the pork is cooked through, transfer to a paper towel-lined plate to drain.
In a small bowl, whisk together the remaining 1/3 cup soy sauce, ½ cup water, sugar, and remaining tablespoon of cornstarch. Set aside.
Discard any fat in the skillet and wipe clean. Heat 1 tablespoon oil over medium heat. Add the lemongrass, garlic, and ginger, and cook, stirring, for 30 seconds. Add the bell pepper and cook for 1 more minute. Stir in the cucumber and cook for an additional 1 minute. Stir in the scallions and ground pork. Pour the prepared sauce into the pan and stir to coat. Cook just another minute, then stir in the crushed red pepper flakes.
Serve hot over the white rice, sprinkled with more crushed red pepper flakes to taste.