1. Slice the pork as thinly as you can (it's easier if you freeze it for 15 to 30 minutes first). Cut the slices into bite-size pieces.
2. Heat a large, heavy skillet over high heat until it begins to smoke. immediately add half the oil and all the pork. Cook, stirring occasionally until the pork browns and loses all traces of pink, about 3 minutes. Transfer the pork to a bowl and lower the heat to medium.
3. Add the remaining oil to the skillet and add the garlic. Stir once or twice. As soon as the garlic begins to color-after about 15 seconds-return the heat to high and add the spinach. Stir frequently, just until the spinach wilts, a minute or two longer.
4. Add the pork back to the skillet and stir for 1 minute. Add the soy sauce and lime juice, stir, turn off the heat, and taste, adding more soy sauce if necessary. If the mixture is drier than you like, add a bit of water and heat through. Garnish with the scallion and serve immediately.