Chop all the vegetables, finely chop the garlic and dice the chicken breast.
Heat olive oil in a large skillet over medium heat. Add the onions and cook for 5–7 minutes until soft and slightly golden.
Add the minced garlic and sliced mushrooms. Cook for 2–3 minutes until fragrant.
Add the chicken pieces and cook for several minutes until lightly browned on the outside.
Add the chopped tomatoes, water, tomato paste, harissa paste, butter, salt, and black pepper. Stir well to combine.
Add the chopped bell peppers and continue simmering uncovered.
As the tomatoes start to break down, add the bell pepper. Carry on cooking until the tomatoes have broken down, the bell pepper chunks have softened up and the excess water has evaporated.
Cook for 10–15 minutes, stirring occasionally, until the sauce thickens and the chicken becomes tender.
Let the dish rest for 5 minutes before serving. Garnish with fresh parsley if desired.