Brown the Chicken: In a large skillet over medium-high heat, add a drizzle of olive oil and heat for about 1 minute. Place the chicken thighs skin-side down and brown them for approximately 5 minutes, until the skin is golden and crispy. Flip and brown the other side for an additional 3 minutes before transferring the chicken to your slow cooker.

Sauté Aromatics: In the same skillet, add the chopped onion and minced garlic to the leftover oil. Sauté over medium heat for about 3 minutes until the onion becomes translucent and fragrant. Be sure to scrape up any browned bits from the chicken for added flavor. Once ready, transfer this aromatic mixture to the slow cooker.

Add Vegetables: Layer in the sliced carrots, thinly sliced celery, and chopped red bell pepper into the slow cooker. Pour in the undrained canned diced tomatoes and the rinsed cannellini beans, ensuring an even distribution of vegetables.

Pour in the Broth: Gently pour the chicken broth over the mixture in the slow cooker, ensuring all ingredients are well submerged. Stir to combine and distribute the flavorful ingredients.

Season the Stew: Sprinkle dried oregano, thyme, rosemary, and a generous pinch of salt and black pepper over the stew mixture. Stir to evenly mix the seasonings.

Slow Cook the Stew: Cover the slow cooker and set it to cook on low for 6 to 7 hours or on high for 3 to 4 hours.

Add Spinach and Cream: About 30 minutes before serving, add the roughly chopped fresh spinach and optional heavy cream. Stir gently to combine.

Final Touches and Serve: Before serving, taste and adjust the seasoning if necessary. Ladle the stew into bowls, topping with freshly grated Parmesan cheese and a sprinkle of chopped basil or parsley.