Heat the olive oil in a large enameled cast-iron casserole. Add the onions and cook over moderately high heat, stirring occasionally, until golden, about 6 minutes. Add the garlic and cook until fragrant, about 3 minutes. Add the okra and cook over moderate heat, stirring, until well coated with the flavorings, about 5 minutes. Add the tomatoes, lemon juice and sugar and season with salt and pepper. Simmer over moderately low heat, stirring occasionally, until the okra is tender and the sauce is thickened, 15 to 20 minutes. Stir in the parsley and season with salt. Serve hot, warm or at room temperature.