Cook the chicken breast in water with a little salt until completely cooked and tender, about 15-20 minutes. Remove from the water and shred with two forks.
Heat the oil in a large skillet over medium heat. Add the chopped onion and cook until soft and lightly golden.
Add the sliced mushrooms and cook until tender and most of their liquid has evaporated.
Add the sliced bell pepper and cook for 2-3 minutes.
Add the shredded chicken, turmeric, garlic powder, salt, and black pepper. Stir well and cook for 3-5 minutes.
Reduce the heat to low. Add the mayonnaise, ketchup, and mustard, then mix until the chicken is evenly coated and creamy.
Lightly toast the baguettes or sandwich rolls.
Fill the toasted bread with the warm chicken mixture and add pickles, lettuce, or cheese if desired.
Serve immediately while warm.