Place chicken in a large bowl with pearl onions, bay leaf, and thyme. Pour the wine over, cover, and refrigerate for 12 to 24 hours.

Strain the marinade into a saucepan, separating chicken and onions. Simmer the wine over medium heat until reduced by half. Pat chicken dry with paper towels.

Preheat oven to 350°F (180°C). Season chicken with salt and pepper. Heat oil in a large Dutch oven over medium-high heat and brown chicken on all sides, about 3 minutes per side. Remove and set aside.

In the same pot, cook bacon until crisp and golden, then remove. Add mushrooms and cook until browned. Remove and set aside. Sauté pearl onions until golden.

Melt butter in the pot. Add garlic and tomato paste, cooking for 1 minute. Stir in flour to form a roux, cooking for 2 minutes. Gradually whisk in the reduced wine and beef stock.

Return chicken, bacon, mushrooms, and onions to the pot. Bring to a simmer, cover, and transfer to the oven. Braise for 45 minutes until chicken is tender and sauce is rich.

Remove from oven and let rest for 10 minutes. Serve over mashed potatoes or tagliatelle, garnished with fresh parsley.