Heat the olive oil in a skillet over medium-high heat. Season the chicken with a little salt and pepper, then give it a quick sear for 2–3 minutes per side until golden. (Totally optional, but that extra browning adds great flavor.)
Add the sliced onion, bell peppers, mushrooms, and garlic to the bottom of your slow cooker.
Layer the chicken right on top of the veggies.
In a large bowl, combine the crushed tomatoes, diced tomatoes, chicken broth, Italian seasoning, basil, oregano, red pepper flakes, salt, and pepper. Give it a quick stir to blend everything together.
Pour the tomato mixture over the chicken and veggies.
Cover and cook on low for 6–7 hours or high for 3–4 hours, until the chicken is tender and easy to pull apart.
Stir in the capers, then taste and adjust seasoning as needed.
Serve the chicken and sauce over pasta, rice, or mashed potatoes. Finish with a sprinkle of fresh parsley and basil before serving.