Start by preheating your oven to 425°F. Pat the chicken thighs dry with paper towels to help them brown more evenly. Season both sides of the chicken generously with salt and freshly ground black pepper.

In a Dutch oven or an oven-safe heavy-bottomed pot, warm the olive oil over medium-high heat. Place the chicken thighs in a single layer, working in batches if necessary to avoid overcrowding. Let them sear undisturbed for 4 to 6 minutes until golden brown on one side. Flip the thighs and cook the other side for an additional 1 to 2 minutes. Once seared, remove the chicken and transfer to a plate—note that the meat will still be undercooked at this stage.

Return the pot to the heat and add the mirepoix mixture along with a small pinch of salt. Sauté for 3 to 5 minutes, stirring occasionally, until the vegetables are softened and beginning to caramelize. Stir in the rosemary, if using, and cook briefly for 30 seconds until aromatic. Pour in the olive brine, using a wooden spoon to deglaze and scrape up any browned bits from the bottom. Let the brine reduce until nearly evaporated, about 1 minute.

Next, toss in the cherry tomatoes and Castelvetrano olives. Season again with a light touch of salt and pepper to balance the briny and sweet elements.

Remove the pot from the stove. Nestle the chicken thighs back into the mixture, seared side facing up, and pour in any juices that have accumulated on the plate. Cover the pot with a tight-fitting lid and place it in the preheated oven. Roast for about 15 minutes, or until the chicken reaches an internal temperature of 170°F to 175°F when measured with an instant-read thermometer.

Serve the chicken warm, generously spooned with the tomato-olive mixture and a drizzle of the savory pan juices.

Store any leftovers in an airtight container and refrigerate for up to four days.