Heat the vegetable oil in a large pot over medium heat. Add the pork shoulder cubes and cook until browned on all sides, about 6 to 8 minutes. Remove the pork and set it aside.
Add the onion, garlic, and lemongrass to the same pot. Cook for 3 minutes until the onion softens and the lemongrass becomes fragrant.
Return the pork to the pot. Pour in the chicken broth and fish sauce. Season with salt and pepper. Bring to a simmer, then cover and cook for 45 minutes until the pork is tender.
Add the carrots and peas. Continue simmering uncovered for 10 minutes.
Stir in the bok choy halves and cook for another 5 minutes until the bok choy wilts but still holds some crunch.
Ladle the stew into bowls and top with fresh cilantro before serving.