Prepare Chicken: Pat the chicken pieces very dry with paper towels. This step is essential for achieving a good sear. Season the chicken lightly with salt and black pepper.

Sear Chicken: Heat the cooking oil in a large cast iron skillet or heavy-bottomed pan over medium-high heat until it shimmers. Add about half of the chicken pieces in a single layer, making sure not to overcrowd the pan. Sear for 2 to 3 minutes per side until golden brown, but do not cook the chicken all the way through. Remove the seared chicken to a plate and repeat with the remaining chicken. The secret to tender chicken is getting a beautiful sear without fully cooking it at this stage, as it will finish cooking gently in the sauce.

Cook Vegetables: Reduce the heat to medium. Add the sliced bell peppers and onion to the same skillet. Cook, stirring occasionally, for 5 to 7 minutes until the vegetables begin to soften and develop a slight char. Stir in the minced garlic and cook for another minute until fragrant.

Build Sauce: Stir in the tomato paste, sugar, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Cook for 1 minute, stirring continuously, to toast the tomato paste and deepen its flavor. Pour in the chicken broth or water, scraping up any browned bits from the bottom of the pan to incorporate them into the sauce. Bring the sauce to a gentle simmer.

Gentle Simmer to Finish: Return all of the seared chicken pieces and any accumulated juices from the plate to the skillet. Stir to coat the chicken evenly with the sauce. Reduce the heat to low, cover the skillet, and let it simmer for 8 to 10 minutes, or until the chicken is cooked through and incredibly fork-tender. This short, covered simmer in the sauce ensures the chicken stays juicy and becomes perfectly tender without drying out.

Serve: Taste and adjust seasoning if needed. Garnish with fresh chopped parsley or cilantro before serving.