In a large Dutch oven or heavy pot, heat the olive oil over medium heat. Add the chicken thighs and cook until browned on both sides, about 5-7 minutes. Remove the chicken and set aside.
In the same pot, add the sliced sausage and cook until browned, about 3-4 minutes. Remove and set aside with the chicken.
Add the onion, garlic, carrot, and celery to the pot. Sauté until the vegetables are softened, about 5 minutes.
Stir in the diced tomatoes, white beans, chicken broth, thyme, rosemary, bay leaf, and season with salt and pepper. Bring to a simmer.
Return the chicken and sausage to the pot, cover, and let simmer for 30 minutes, stirring occasionally.
Preheat the oven to 400°F. After 30 minutes, remove the bay leaf and sprinkle the breadcrumbs over the top of the cassoulet.
Transfer the pot to the oven and bake for 15-20 minutes, or until the breadcrumbs are golden and crispy.
Remove from the oven, let cool for a few minutes, and garnish with fresh parsley before serving.