In a large skillet, heat 2 tablespoons oil over medium high heat until shimmery. Add chicken and onion to skillet and saute several minutes until chicken is no longer pink. Add garlic to skillet and saute an additional minute until garlic is fragrant. Transfer chicken to a bowl and set aside.
In same skillet over medium-high heat, heat remaining 1 tablespoon oil until shimmery. Saute broccoli, sugar snap peas, and red pepper 5-8 minutes until vegetables are crisp-tender. Add chicken back to skillet and reduce heat to medium-low.
In a small bowl, whisk peanut butter, soy sauce, vinegar, sugar, and cornstarch mixture until smooth. Add to chicken mixture and let stir-fry cook over medium-low heat, stirring occasionally until sauce is thickened and bubbly.
Add parsley to stir-fry and season with salt and pepper to taste. Serve hot over rice and enjoy!