Heat 1 teaspoon olive or canola oil in a large skillet over medium heat.
Add 1/2 cup diced red onion and cook, stirring occasionally, until slightly caramelized, about 5 minutes.
Add 1 cup rinsed and drained black beans, 1/2 teaspoon paprika, 1/2 teaspoon cumin, and 1/4 teaspoon kosher salt; stir and cook until the beans are heated through, 3–4 minutes. Transfer the bean mixture to a medium bowl and set aside.
Increase the skillet heat to medium-high. Add the 38 ounces diced boneless skinless chicken breasts (1/2-inch cubes) in a single layer. Cook, turning occasionally, until the chicken is browned and cooked through, about 5–8 minutes (internal temperature 165°F / 74°C).
Transfer the cooked chicken to a medium bowl and toss with 1/2 cup buffalo sauce until evenly coated.
Divide the 3 cups cooked brown rice among bowls (about 3/4 cup per bowl if making four). Top the rice in each bowl with the reserved bean mixture and evenly sprinkle the 1/2 cup part-skim cheddar cheese over the bowls.
Divide the buffalo chicken among the bowls on top of the beans and cheese.
If serving immediately, microwave each bowl about 30 seconds to melt the cheese. Top each bowl with the 1/4 cup chopped scallions and serve.
Refrigerate leftovers in an airtight container for up to 4 days. To reheat, microwave about 3 minutes or until heated through.