Season your diced chicken with salt and pepper. Heat oil in a wok or skillet and cook the chicken until golden and fully cooked, about 6-8 minutes. Transfer to a bowl and set aside.
Whisk together fish sauce, ketchup, brown sugar, peanut butter, garlic, and lime juice in a small bowl. Set aside; this sauce is the heart of your Pad Thai.
In the same wok, pour in the beaten eggs and let them cook for a minute, then stir to scramble. Add sliced onions and bean sprouts, cooking for another 1-2 minutes until slightly softened.
Return the chicken to the pan, pour in the sauce, and stir to coat everything evenly. Add the cooked noodles and toss until noodles are fully coated with the sauce.
Stir in chopped green onions and cilantro. Sprinkle with chopped peanuts for crunch. Serve immediately while warm.