Bring a large pot of water to a boil over high heat. Stir in the spaghettini and return to a boil. Cook the pasta until it has cooked through but is still firm to the bite, about 6-8 minutes. Drain well and keep warm.
Heat 2 tablespoons oil in a large frypan over medium heat. Stir in the tomatoes; cook until they soften and begin to break down. Stir in the garlic powder, oregano, basil, salt, pepper, sugar and tomato sauce. Heat sauce through and reserve.
Heat the remaining 3 tablespoons oil in a separate cast iron frypan over medium heat. Stir in chicken; cook until browned. Stir in crushed garlic cloves; cook for 1 additional minute.
Remove chicken from frypan and reserve. Turn heat to high. Stir the green capsicum, red capsicum, onion and mushrooms into the frypan and cook until they begin to soften. Stir in browned chicken. Turn heat to medium and cook until chicken is no longer pink in the centre and the vegetables are cooked through, about 5 minutes.
Toss the chicken and vegetables with the tomato sauce and the hot pasta. Serve sprinkled with Parmesan cheese.