In a non-stick pan, arrange the chicken thighs in a single layer, skin side down. Cook over high heat until the skins render fat and are lightly browned. Flip the chicken thighs over and lightly brown the other side as well.
Add the garlic, onion and stewed tomatoes, crushing the whole tomatoes with your hand. Season with salt and pepper. Bring to the boil and cook, uncovered, until the liquid is slightly reduced. Lower the heat, cover and simmer for about 15 minutes.
Uncover the pan, add the basil leaves and quartered portobello mushrooms. Cover and continue cooking over medium heat for another five minutes.
Garnish with torn fresh parsley and serve hot.