Season the chicken with salt and black pepper. Heat the oil in a wok over medium-high heat. Add the chicken and stir-fry until brown and cooked through, about 3 minutes. Set aside.
Return the wok to medium-high heat. You should have a good bit of oil left- if not, add a some more. When the oil is hot, add the lemongrass, onion and seranno pepper. Stir-fry until the onion is soft, about 1 minute.
Push the mixture to one side of the wok, then add the eggs to the other side. Stir-fry, breaking up the eggs, until cooked through, about 30 seconds. When the eggs are set, stir everything together.
Return the chicken to the pan, add the bell pepper, and stir. Add the fish sauce, lemon zest and juice, tamarind concentrate and noodles. Stir and cook until heated through, about 2 minutes.
Transfer to plates and top with peanuts, cilantro, and bean sprouts.