Combine the SunButter, tamari, coconut sugar, rice vinegar, and cornstarch slurry in a small bowl. Stir until smooth and set aside.
Heat 2 tablespoons oil in a large skillet over medium high heat. Add the chicken and cook until no longer pink, about 5 minutes. Remove from the skillet, add the other tablespoon of oil and let it heat. Add the garlic, stir fry vegetables, red pepper, and cashews if using. Cook, stirring occasionally, until the vegetables are crisp tender, about 5 minutes. Reduce heat to medium-low. Return the chicken to the pan along with the sauce. Heat, stirring occasionally, until the sauce is hot and thickened, about 2 minutes. Season to taste with salt and pepper.