Season lamb cubes with sea salt and pepper. Heat oil in 5-quart Dutch oven on medium-high heat. Add 1/2 of the lamb cubes; cook 4 to 5 minutes or until browned on all sides. Remove lamb from pan. Repeat with remaining lamb cubes. Add onions and garlic to pan; cook and stir on medium heat 5 to 8 minutes or until onions are softened. Sprinkle with flour. Cook and stir 1 minute.
Stir wine into pan. Bring to boil, stirring to release browned bits in bottom of pan. Return lamb to pan. Add broth, tomatillos, rosemary, thyme, cumin and bay leaf. Return to boil. Reduce heat to low; cover and simmer 1 1/2 hours.
Remove lamb from pan. Bring sauce to boil on high heat. Cook 10 minutes or until sauce is thickened. Skim fat from sauce. Return lamb to sauce; cook until heated through.
Ladle stew into serving bowls. Top with a dollop of sour cream and cilantro.