Using a food processor, coarsely chop the onion and bell pepper.
In a large nonstick skillet, heat the EVOO over medium-high heat. Add the onion-pepper mixture and cook until softened, about 3 minutes.
Add the bison, breaking it up with a spoon, and cook until it loses its redness. Stir in the chili powder. Add the tomato puree, brown sugar, vinegar, worcestershire and salt. Simmer until the sauce thickens, about 5 minutes. Spoon over the bun halves and serve open-face.