Heat a large heavy wok over high heat. When it is hot, add oil, and heat until very hot. Add garlic, and stir-fry until just golden, about 20 seconds. Add pork, if using, and cook, stirring constantly, until all the pork has changed color completely, about 1 minute.
Add rice, breaking it up with wet fingers as you toss it into wok. With your spatula, keep moving rice around wok. At first it may stick, but keep scooping and tossing it, and soon it will be more manageable. Try to visualize frying each little bit of rice, sometimes pressing the rice against the wok with the back of the spatula. Good fried rice should have a faint seared-in-the-wok taste. Cook for about 30 seconds. Add the greens, then the fish sauce, and stir-fry for 30 seconds to 1 minute.
Turn out onto a dinner plate, and garnish with coriander, cucumber slices, scallion, and lime wedges. Squeeze lime onto rice as you eat it, along with chile sauce -- the hot, salty taste of the sauce brings out the full flavor of the rice.