Add the olive oil and sausages to a large Dutch oven set over medium-high heat and sear until browned, about 3 minutes. Remove the sausages to a plate and set aside. Sprinkle the chicken thighs on both sides with salt and pepper, then sear the chicken in the Dutch oven, 4 to 5 minutes per side; remove to a plate and set aside.
Now, add the mushrooms, pearl onions, red peppers, roasted garlic and pepper flakes to the pot; sprinkle with salt and pepper. Saute until the vegetables are softened and the pearl onions translucent, 4 minutes. Add the wine, bring to a boil, and then simmer for 5 minutes, deglazing the pan.
In a large roasting pan add the browned sausages and chicken thighs, the sauteed vegetables, and the chicken stock and marinara sauce. Tie the thyme and rosemary into a bundle using butcher's twine and add it to the pan. Stir all the ingredients together and transfer the roasting pan to the oven. Cook until the chicken thighs are cooked through and tender and the sauce has thickened, 35 to 45 minutes. Remove the herb bundle.
Serve the cacciatore on a large serving platter, garnished with parsley.