Preheat the grill to medium-high heat. In a small bowl, mix olive oil, garlic powder, onion powder, smoked paprika, salt, and pepper. Rub this mixture over the chicken breasts.
Grill the chicken for about 6-7 minutes on each side or until the internal temperature reaches 165°F. Remove from the grill and let it rest for 5 minutes before slicing.
While the chicken is grilling, prepare the chimichurri sauce by combining parsley, cilantro, garlic, red pepper flakes, olive oil, red wine vinegar, salt, and pepper in a bowl. Mix well and set aside.
In a large bowl, layer the cooked brown rice, mixed greens, cherry tomatoes, corn, and diced avocado.
Slice the grilled chicken and place it on top of the salad. Drizzle with chimichurri sauce before serving.