Pat the chicken thighs dry with paper towels. Season both sides evenly with the kosher salt and black pepper.

Heat 2 teaspoons of olive oil in a large skillet over medium-high heat. Once the oil is hot and shimmering, carefully place the seasoned chicken thighs in the skillet, ensuring not to overcrowd the pan (work in batches if necessary). Sear the chicken for 3-4 minutes per side until well-browned. Transfer the browned chicken pieces to the insert of your slow cooker (a 4-quart size or larger is recommended).

Lower the heat under the skillet to medium. Add the remaining 1.5 teaspoons of olive oil. Add the chopped onion and cook, stirring occasionally, for about 3-4 minutes until it starts to soften.

Stir in the minced garlic and cook until fragrant, about 30 seconds more. Pour in the 2 teaspoons of balsamic vinegar, using a spoon or spatula to scrape up any flavorful browned bits stuck to the bottom of the skillet. Cook for another 30 seconds. Carefully transfer this onion-garlic mixture to the slow cooker.

Pour the can of crushed tomatoes over the chicken in the slow cooker. Top with the chopped green bell pepper, sliced mushrooms, Italian seasoning, dried oregano, and the bay leaf. Stir gently just to combine the sauce ingredients around the chicken pieces.

Secure the lid onto the slow cooker. Cook on the LOW setting for 5 to 6 hours or on the HIGH setting for 3 to 4 hours. The chicken should be very tender and cooked through, reaching an internal temperature of 165°F (74°C).

Before serving, carefully remove and discard the bay leaf. You can serve the chicken thighs whole, or remove them to a cutting board, shred or chop the meat, and then return it to the sauce in the slow cooker to mix.

Taste the sauce and adjust seasoning with additional salt or pepper if desired.

Serve the rustic Italian chicken hot, spooning plenty of the chunky tomato and vegetable sauce over each portion. Garnish with chopped fresh parsley and grated Parmesan cheese, if using.