In a large dutch oven, over medium high heat, heat the olive oil. Dredge the chicken in the flour using a resealable plastic bag. Brown the chicken in the oil for about 5 minutes per side. Add the onions to the chicken and continue to cook, stirring occasionally until soft (about 5 minutes). Add the garlic and cook for about 30 seconds. Add the chicken broth, lemon zest and juice, tomatoes and juice, and oregano. Bring to a boil and then reduce the heat to low. Add the olives, cover and simmer for about 45 minutes or until the chicken is very tender and starts to fall off the bone. Adjust the seasoning with salt and pepper. Serve with the crumbled feta, if desired.