Heat 2 tablespoons olive oil in a large skillet over medium-high heat.
Rub the 6 boneless skinless chicken breasts with 1 tablespoon cajun seasoning. Add the chicken to the hot skillet and cook 3–4 minutes per side, until browned. Remove the chicken and set aside on a plate; tent with foil to keep warm.
Add 2 cups long grain rice, 3 cups chicken broth, 1 (15-ounce) can drained black beans, 1 (15-ounce) can diced tomatoes with juice, 1 teaspoon chili powder, 1/4 teaspoon paprika, and 1/4 teaspoon cumin to the skillet. Stir to combine and scrape up any browned bits from the bottom of the pan.
Bring the mixture to a boil, then reduce heat to low. Cover and simmer 20–25 minutes, until the liquid is absorbed and the rice is tender.
Nestle the reserved chicken into the rice, cover, and cook until the chicken is heated through and registers 165°F on an instant-read thermometer.
Garnish with fresh chopped parsley and sliced avocado, if desired, and serve.