In a small nonstick skillet, toast the Szechuan peppercorns over moderately high heat until just fragrant and beginning to darken, about 5 minutes. Transfer to a spice grinder and let cool, then grind to a powder.
In a large skillet or wok, heat 1/4 cup of the oil until shimmering. Add the pork and stir-fry over moderately high heat until just cooked through, about 5 minutes. Add the ginger, garlic, scallions and half of the Szechuan peppercorn powder and stir-fry until the scallions are beginning to soften, about 2 minutes. Transfer the mixture to a bowl.
Return the skillet to moderately high heat. Add the remaining 1/4 cup of oil and heat until shimmering. Add the green beans and fry until they begin to blister and crisp up, about 7 minutes. Add the stock and simmer until the beans are just tender and the liquid has evaporated, about 7 minutes longer.
In a small bowl, whisk the soy sauce with the fish sauce, sake and sugar. Pour the sauce over the beans. Add the pork mixture and toss until the beans are coated and the pork is hot, about 2 minutes. Transfer the beans to a platter, dust with the remaining Szechuan peppercorn powder and serve hot.