Smack whole cucumbers, turning against work surface 3-4 times to loosen and tenderize. (Bruising the cucumbers helps break down their flesh, which is then better able to absorb the flavor of the marinade.) Slice cucumbers into 3"-long pieces; set aside. Whisk soy sauce, cornstarch, and ½ tsp. pepper in a large bowl; mix in pork and set aside.
Heat oil in a large skillet over medium heat. Cook onion, garlic, and ginger, stirring often, until softened, about 5 minutes; mix in turmeric. Add pork mixture and cook, breaking up with a spoon, until brown and cooked through, about 5 minutes. Increase heat to high. Add vinegar, chiles, and smacked cucumbers. Toss to coat. Transfer to a medium bowl and let sit, tossing occasionally, 1 hour; season with salt and pepper.