In a 5-to 6-quart Dutch oven, cook the bacon in the oil over medium until browned but not too crispy, about 5 minutes. Using a slotted spoon, transfer the bacon to a small bowl. Season the chicken with salt and pepper. Increase the heat to medium-high. Working in batches, cook the chicken until golden brown, about 5 minutes per side. Transfer the chicken to a plate.
Add the onions and mushrooms to the pot and cook, stirring occasionally, until they begin to soften, about 2 minutes; season. Stir in the tomato paste, thyme and garlic. Add the wine and cook, scraping up any browned bits, until wine is reduced by half, about 5 minutes. Add the stock and bring to a simmer. Return the chicken and any juices to the pot. Reduce the heat to low, partially cover and gently simmer, turning the chicken once or twice, until the chicken is cooked through and the onions are tender, about 40 minutes.
In a small bowl, dissolve the cornstarch in 3 tbsp. water. Transfer the chicken to a plate. Increase the heat to medium-high and bring the cooking liquid to a boil. Add the cornstarch mixture and stir until thickened, about 3 minutes; season. Return the chicken and bacon to the pot, stirring to coat. Serve in shallow bowls with crusty bread.