Start by cutting the boneless, skinless chicken breasts into bite-sized pieces. This will help them cook evenly and absorb more flavor. Season the chicken with salt, black pepper, and garlic powder.
In a large skillet or Dutch oven, heat a drizzle of olive oil over medium heat. Add the diced onion and green bell pepper. Sauté for about 5 minutes, until the onion is translucent and the pepper is tender. Next, add the diced mushrooms and pressed garlic, cooking for another 2-3 minutes until fragrant.
Push the vegetables to the side of the skillet and add the seasoned chicken pieces. Brown the chicken for about 5 minutes on each side, until golden brown.
Once the chicken is browned, pour in the crushed tomatoes, stirring everything together. Bring the mixture to a gentle simmer.
Reduce the heat to low, cover the skillet, and let it simmer for about 30 minutes.
Once the chicken is cooked through and tender, remove the skillet from heat. Stir in the freshly grated parmesan cheese until melted and well incorporated. Serve hot, garnished with freshly chopped parsley if desired.