Toss the sliced pork with the cornstarch and 1 teaspoon fish sauce. Leave it for 10 minutes while you prep everything else.
Heat 1 tablespoon oil in a wok or large skillet over high heat until shimmering. Add the pork in a single layer and sear for 1 minute without moving, then stir for 1 to 2 minutes until just cooked through. Move it to a plate.
Lower the heat to medium and add the remaining oil. Add the curry paste and fry for 1 minute, pressing it into the oil, until it smells fragrant and the oil turns red.
Add the green beans and 2 tablespoons water. Stir fry for 3 to 4 minutes until the beans are bright and tender with a slight bite.
Return the pork to the pan with 1 tablespoon fish sauce and the sugar. Toss for 1 minute over high heat until the sauce clings and there is no liquid left in the pan.
Turn off the heat, stir in most of the shredded lime leaves, and serve over rice with the remaining lime leaves on top.