Pat your chicken thighs dry with paper towels and season them generously with salt and black pepper. Chop your yellow onion, green bell pepper, and red bell pepper into bite-sized pieces. Slice the cremini mushrooms and mince the garlic cloves.

Heat the olive oil in a large skillet over medium-high heat. Once shimmering, add the seasoned chicken thighs and sear for 3-4 minutes per side, until deeply golden brown. Don't worry about cooking them through; we're just building flavor here! Remove the chicken from the skillet and set aside.

In the same skillet (no need to clean it!), add the chopped onion, bell peppers, and sliced mushrooms. Sauté for 5-7 minutes, stirring occasionally, until the vegetables begin to soften. Stir in the minced garlic and cook for another minute until fragrant.

Transfer the sautéed vegetables to the bottom of your 6-quart slow cooker. Place the seared chicken thighs on top of the vegetables. Pour in the crushed tomatoes and chicken broth (or red wine, if using). Sprinkle in the dried oregano, dried basil, the remaining 0.5 tsp salt, and 0.25 tsp black pepper. Give it a gentle stir to combine, making sure the chicken is nestled into the sauce.

Cover the slow cooker and cook on Low for 6-8 hours, or on High for 3-4 hours, until the chicken is fork-tender and pulls apart easily.

Once cooked, you can either shred the chicken directly in the slow cooker with two forks or leave the thighs whole. Taste the sauce and adjust seasonings if needed. Ladle the chicken and sauce over your favorite pasta, polenta, or rice. Garnish generously with fresh chopped parsley before serving.