Preheat your oven to 425°F. Pat the chicken thighs dry with paper towels and season them generously with salt and pepper.
Heat the olive oil in a Dutch oven or a large oven-safe skillet over medium-high heat. Once hot, add the chicken thighs in a single layer. Cook for 4-6 minutes on one side, flip, and cook for another 1-2 minutes. Remove the chicken from the pan and set aside. (It won't be fully cooked yet.)
Add the mirepoix to the same pan, along with a pinch of salt. Sauté for 3-5 minutes, stirring occasionally, until softened and lightly browned.
If using, stir in the rosemary leaves and cook for about 30 seconds until fragrant.
Deglaze the pan by adding the olive brine, scraping up any browned bits with a wooden spoon. Cook until the brine evaporates, about 1 minute.
Stir in the cherry tomatoes and olives. Season to taste with more salt and pepper.
Return the chicken thighs to the pot, placing them seared side up. Pour in any accumulated juices from the resting plate.
Cover the pot with a lid (or tightly with foil) and bake in the preheated oven for 15 minutes, or until the chicken reaches an internal temperature of 170°F to 175°F on an instant-read thermometer.
Serve the chicken thighs topped with the olive-tomato mixture and enjoy with your favorite grain or vegetable side dish!