Using paper towels, pat the chicken pieces dry; season liberally with salt and pepper. In a Dutch oven, brown chicken in oil over medium high heat, about 5 minutes per side; transfer to a platter.
Reduce heat to medium and cook onions and garlic, scraping up any browned bits, 3 to 5 minutes; add in bay leaf and thyme.
De-glaze pot with vinegar and simmer until reduced by half, about 5 minutes. Pour in chicken stock and bring to a boil; add greens, stirring until wilted, about 30 seconds.
Return chicken pieces, skin side up, and any accumulated juices, to pot until cooked through, about 10-12 minutes (occasionally baste with juices).
Transfer chicken to a serving platter and bring sauce to another boil; reduce heat to medium low and stir in creme fraiche. Cook, while stirring until sauce thickens slightly, about 3 minutes. Spoon sauce and greens over chicken; serve with additional creme fraiche, if desired.