Preheat oven to 400 F.

In large skillet, heat oil on medium heat. Add diced onion and cook until translucent and soft, about 5 minutes. Stir in tomato paste and garlic and cook until garlic becomes fragrant, about 1 to 2 minutes. Add bison and brown, seasoning with salt and pepper, to taste. Once browned, add in diced tomatoes and black beans. Add chili powder, oregano, and cumin and stir to combine. Remove from heat and set aside.

Cut peppers in half lengthwise and carefully scoop out core and seeds. Place in 9 x 11 inch (23 x 28 cm) baking pan, cut side up. The bottoms of the peppers may not be flat so prop the peppers against sides of pan and nest to each other. Spoon bison/black bean mixture into peppers. Bake filled peppers for 25 minutes, covered. Remove from oven, and sprinkle each pepper with shredded cheese; return to oven, uncovered, for 10 minutes, until cheese is bubbly and starts to color. Remove from oven and garnish with chopped cilantro, if using.