in large skillet heat oil. add the chicken and brown 2 minutes on each side. Transfer to plate

in some skillet, cook onion and garlic, stirring as needed until softened, about 5 minutes

add the mushrooms, tomatoes, bell pepper, tomato paste, oregano, and black pepper. bring to boil, stirring as needed.

return chicken to skillet.....reduce heat and simmer covered stirring as needed until chicken is cooked through, about 25 minutes

serve the spaghetti topped with chicken and sauce.