Heat the broiler, if using to char the bell peppers.
- Peel and coarsely chop the onion.
- Cut a small corner from the ground turkey packaging and drain off any excess liquid. Transfer to a plate; pat dry with a paper towel.
- Rinse the black beans.
In a large frying pan over medium-high heat, warm 1 to 2 tablespoons oil until hot but not smoking. Stir in the onion, season with salt and pepper, and cook until starting to soften, 1 to 2 minutes. Stir in the Southwestern spice blend and cook until fragrant, about 1 minute.
Add the turkey, season with salt and pepper, and cook, stirring to break up the meat, until lightly browned but not yet cooked through, 3 to 4 minutes. Add the beans and tomatoes and cook, stirring occasionally, until the turkey is cooked through and the beans are warm, 2 to 3 minutes. Remove from the heat and season to taste with salt and pepper.
While the filling cooks, prepare the peppers.