Season 1 ½ pounds skinless chicken thighs with salt and black pepper. Heat 3 tablespoons olive oil in a large non-stick skillet over medium-high heat. Cook chicken until golden brown, about 3 minutes per side. Transfer chicken to slow cooker. Alternatively, skip browning and add chicken straight to a 6-quart slow cooker if pressed for time.
Add 4 teaspoons minced garlic, ½ cup chopped white onion, ½ cup diced red bell peppers, ½ cup sliced carrots, ½ cup sliced mushrooms, 1 (28-ounce) can diced tomatoes with green chilis, 1 (6-ounce) can tomato paste, 1 teaspoon dried basil, ½ teaspoon dried oregano, ¼ teaspoon dried rosemary, ¼ teaspoon dried thymeand salt and black pepper to taste. Stir to combine. Do not worry if the mixture looks dry, the tomatoes will release more liquid as this cooks - if you like this dish more saucy, add ½ cup chicken broth or water.
Cover and cook on high for 3-4 hours or on low for 4-6 hours.
In a small bowl, stir together 1 tablespoon water and 2 teaspoons cornstarch. Add to the crockpot along with ¼ cup Kalamata olives, and ½ cup chopped kale. Cook for an additional 20-30 minutes on HIGH, or until the sauce thickens slightly and the kale is wilted.
Sprinkle with 2 tablespoons fresh chopped parsley, and fresh basil leaves. Taste and season with additional salt or black pepper, as needed.
Serve hot over pasta, zucchini noodles, rice, or your favorite sides, and top with grated Parmesan cheese, if desired.