In a large Dutch oven, heat the oil over medium-high heat. Add lamb dredged in flour to hot oil in batches until the lamb is browned well on all sides, 7-10 min. Transfer the browned lamb to a plate.
Add another T of oil to the pan and add the garlic and onions. Cook, stirring, until the onions are soft, about 8 min. Pour in the wine to deglaze the pan, about 3 min.
Return the lamb to the pan along with any juices on the plate. Add the stock, tomatillos, chopped cilantro, thyme, and bay leaf. Bring to a boil, reduce the heat to low, cover. Cook for about 1 ½ hr, stirring occasionally, until tender.
Transfer the lamb to a serving bowl. Reduce the sauce over high heat until thickened, about 10 min. Strain the sauce over the meat, discarding the solids.
Return the meat and sauce to the Dutch oven. Garnish each serving with a dollop of crème fraiche.