Preheat the oven to 375°F (190°C).
Place the chicken breasts flat in a large baking dish so they are not touching.
Sprinkle the taco or fajita seasoning evenly over the chicken.
Toss the sliced bell peppers and sliced yellow onion together in a bowl, then spread them evenly over the seasoned chicken.
Sprinkle the shredded cheddar cheese evenly over the peppers and onions.
Bake on the middle rack for 30–40 minutes, until the chicken reaches an internal temperature of at least 165°F (74°C) and is cooked through.
If you prefer less-crispy cheese, add the cheese during the last 5–8 minutes of baking.
Remove from the oven, garnish with chopped cilantro if using, and serve hot over rice, cauliflower rice, greens, or in tortillas.