1Prepare all ingredients before cooking as this stir-fry comes together very quickly. Slice the pork or chicken into thin strips, about 1/4 inch thick and 2 inches long - thin slices ensure quick cooking and maximum surface area for the curry paste to coat. Trim the ends off the green beans and cut them in half, or into 2-inch pieces. Mince the garlic. Remove the tough center stem from the kaffir lime leaves, stack them, roll tightly, and slice into very thin ribbons (chiffonade) - this releases their aromatic oils and makes them pleasant to eat. Mix together the fish sauce, oyster sauce, palm sugar, and chicken broth in a small bowl, stirring until the sugar dissolves.
2Heat a wok or large skillet over high heat until very hot. Add the vegetable oil and swirl to coat. Add the green beans and stir-fry for 3-4 minutes, tossing frequently, until they are blistered in spots, bright green, and tender-crisp. The beans should have some charred spots which add flavor but still have a nice snap when bitten. If your wok is not large enough to cook them in a single layer, work in batches. Transfer the beans to a plate and set aside.
3Return the wok to high heat. Add a bit more oil if needed. Add the minced garlic and stir-fry for just 10-15 seconds until fragrant. Immediately add the red curry paste. This is the key step - stir-fry the curry paste vigorously for 1-2 minutes until it becomes very fragrant, darkens slightly, and the oil begins to separate and pool around the edges. The paste should sizzle and transform from a wet mixture to a dry, aromatic coating. This frying process is what makes pad prik king a dry curry rather than a saucy one. Be careful not to burn the paste - reduce heat slightly if it begins to scorch.
4Add the sliced pork or chicken to the wok with the fried curry paste. Stir-fry vigorously for 2-3 minutes, tossing and flipping constantly, until the meat is coated in the red curry paste and cooked through. The curry paste should coat each piece of meat evenly, creating a vibrant red color. If the paste sticks to the wok, add a splash of the sauce mixture to help deglaze. The meat should be just cooked through - do not overcook or it will become tough.
5Return the blistered green beans to the wok. Pour in the prepared sauce mixture. Toss everything together over high heat for about 1 minute until the sauce is absorbed and the beans are reheated. The dish should look dry and glossy, with the curry paste coating all the ingredients rather than pooling as a sauce. Add the sliced red chili if using for extra heat and color. Taste and adjust seasoning - add more fish sauce for saltiness, more sugar for sweetness, or more curry paste for heat.
6Remove the wok from heat and add the thinly sliced kaffir lime leaves. Toss briefly to distribute - the residual heat will release their incredible citrus aroma without cooking them. The lime leaves should remain bright green and fragrant. Transfer immediately to a serving plate. The finished dish should be dry and glossy, deeply red from the curry paste, with visible green beans and ribbons of kaffir lime leaf throughout. Serve hot with steamed jasmine rice. Pad prik king is also delicious at room temperature, making it suitable for Thai lunch boxes and packed meals. The flavors actually improve slightly as they meld, though the green beans will lose some crispness. Store leftovers refrigerated for up to 3 days and reheat in a wok or skillet.