whisk soy sauce, peanut butter and 1/4 cup water until smooth. In large skillet, heat 2 tablespoons oil over medium heat. Add vegetables, mushrooms, and pepper flakes, cook until vegetables are softened. Stir in chicken and sauce mixture until combined and hot, 1-2 minutes. Serve over rice. If desired, sprinkle with chopped peanuts and sliced scallions.