Season the chicken. Pat the chicken pieces dry with paper towels, then season on all sides with 2 tsp Kosher salt and ¼ tsp black pepper.
Brown the chicken. Heat ¼ cup of olive oil in a large skillet over medium-high heat. When hot, and working in batches, add the chicken pieces and cook until browned on all sides, about 10-12 minutes. Rotate the chicken every few minutes so that it’s evenly browned. Transfer the chicken to a plate. If the pot looks a little dry, add 1-2 Tbsp more olive oil (it should be fine, but you don’t want the veggies to stick!).
Sauté the aromatics. To the same pot, add 1 chopped onion, 2 cups thinly sliced mushrooms, diced red and yellow bell peppers, and 4 oz diced pancetta. Cook, stirring often, until the onions are softened and the pancetta is beginning to crisp, about 6-8 minutes. Season with remaining ½ tsp Kosher salt, then add 3 cloves chopped garlic, fresh sage leaves and rosemary sprigs. Cook until fragrant, about 1 minute more.
Make the sauce. Pour in ½ cup dry white or red wine, then bring to a boil. Allow the mixture to reduce by half, about 5-6 minutes, then pour in 1 (28oz) can of whole peeled San Marzano tomatoes (or crushed tomatoes). You can either break them up with your hand as you add them to the pot or use a spatula. Add a pinch of sugar, then return the chicken, along with any juices on the plate, to the pot.
Simmer. Cover with a lid, reduce the temperature to medium-low, then simmer for 25 minutes, stirring once or twice.
Finish, then serve! Once the chicken is fully cooked and the sauce has reduced substantially, remove the lid. Stir in ⅓ cup black olives, then taste the sauce and adjust seasoning. Transfer to a serving platter, then garnish with fresh basil and a pinch of flaky sea salt. Enjoy!