Heat 1 teaspoon oil in a medium pot over medium-low heat.
Add the diced red onion and cook, stirring occasionally, until slightly caramelized, about 5 minutes.
Stir in the black beans, 1/2 teaspoon paprika, 1/2 teaspoon cumin, and 1/4 teaspoon kosher salt; cook 3–4 minutes until heated through.
Meanwhile, heat a large skillet over high heat and lightly coat with cooking spray or a little oil.
Add the diced chicken to the hot skillet and cook, flipping occasionally, until browned and cooked through, about 5 minutes.
Transfer the cooked chicken to a medium bowl and toss with the 1/2 cup buffalo sauce until evenly coated.
Divide 3/4 cup cooked brown rice among four bowls. Top each with 1/4 cup the bean-onion mixture and 1/8–1/4 cup cheddar cheese, then divide the buffalo chicken evenly over the bowls.
If desired, microwave bowls about 30 seconds to melt the cheese, then sprinkle with chopped scallions and serve.