1Place the sherry or rice wine, 3 tablespoons of the soy sauce, the brown sugar, and 1 tablespoon of the cornstarch in a small bowl and whisk to combine; set aside.
2Slice the pork against the grain into even, paper-thin pieces and place in a large bowl. (It doesn’t matter how wide the slices are as long as they are even in thickness.) Add the remaining 1 tablespoon soy sauce, the remaining 1 tablespoon cornstarch, the five-spice powder, and 1 tablespoon of the oil and toss to evenly coat the pork. Set aside to marinate for 5 to 10 minutes.
3In a large nonstick frying pan with a tightfitting lid, heat the remaining 2 tablespoons oil over medium-high heat. When the oil just begins to smoke, add the pork and cook, stirring rarely, until browned and almost completely cooked through, about 2 minutes. Remove to a plate and set aside.
4Reduce the heat to medium and add the sweet potato and water to the pan. Cover and cook until the sweet potato is knife tender, about 3 to 5 minutes. Add the spinach, cover, and cook until it just begins to wilt, about 30 seconds.
5Remove the cover, increase the heat to high, add the snap peas, garlic, and ginger, and cook, stirring occasionally, until the snap peas are bright green, about 2 minutes. Whisk the reserved soy sauce mixture again until evenly combined and add it to the pan along with the reserved pork and any accumulated juices. Cook, stirring continuously, until the sauce has thickened and the pork is cooked through, about 30 seconds. Sprinkle with the scallions, stir, and serve immediately, with rice if desired.