Cut chicken into serving size pieces. Saute in hot oil until browned. Remove from pan, drain all but a little oil from pan. Add garlic and shallots; cook 1 minute. Add wine and return chicken to pan. Cover and simmer until tender, about 15 minutes. Add more wine if needed. Increase heat, add leeks, toss and cook briefly. Season with salt and pepper. Cover and cook over medium heat for 3 minutes. Stir in mustard and cream. If sauce is too thick, thin it with a little wine.