Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil.
Bring a large pot of salted water to a boil; cook peppers until just soft, 6 to 7 minutes. Drain and transfer to the prepared baking sheet.
Heat oil in a skillet over medium heat; add chicken and 1 1/2 teaspoon taco seasoning mix. Cook and stir until chicken is no longer pink in the center, about 8 minutes. Chop or shred chicken and transfer to a bowl.
Cook and stir onion and garlic in the same skillet over medium heat until tender, 5 to 10 minutes.
Mix onion mixture, black beans, tomato sauce, and cauliflower rice into the bowl of chicken. Stir in 1 1/2 tablespoons taco seasoning. Fill the bell pepper halves with the chicken mixture.
Bake in the preheated oven until heated through, 10 to 15 minutes. Top chicken mixture with Cheddar cheese and continue baking until cheese is melted, about 5 minutes more.
Mix Greek yogurt, cilantro, lime juice, and 1/2 teaspoon taco seasoning together in a bowl; serve over stuffed peppers.
