Pat the chicken thighs or breasts dry with paper towels, season both sides generously with salt and black pepper.

Heat olive oil in a skillet over medium-high heat, add the seasoned chicken, searing until golden brown on both sides, about 5-7 minutes. Remove chicken and set aside.

In the same skillet, add chopped onion and sauté until caramelized, about 5 minutes.

Stir in minced garlic and red pepper flakes, and sauté for an additional minute.

Pour in diced tomatoes, return the browned chicken to the skillet ensuring well-coated in sauce.

Reduce the heat to medium-low, cover and simmer for about 15 minutes, or until chicken reaches an internal temperature of 165°F.

Remove from heat and sprinkle fresh parsley over the dish. Serve immediately.