Mix the chicken, soy sauce, and garlic salt in a medium bowl.
Heat the oil in a large skillet over medium-high heat. Add the chicken mixture and cook through. Remove the chicken from the skillet.
Add the peanut butter and 2/3 of the chopped scallions to the skillet and cook together for 1 to 2 minutes. Add the chicken back to the skillet and heat through.
Serve chicken on top of spaghetti and garnish with additional scallions.