Peel and cube eggplant. Soak 15 minutes in salted water and drain well. Heat butter in soup kettle, add eggplant, stir, cover, and cook 20 minutes over very low heat. Stir several times as eggplant cooks. Add tomatoes, onions, garlic, and bay leaf. Cover again and cook 10 minutes, stirring occasionally until vegetables are well blended. Add rice and stock. Cover and simmer 30 minutes. Season with salt and pepper. Garnish with chopped chives.