Let the beans soak in water for 8 hours. Boil them in a pot for 25 minutes; strain and rinse them under tap water. Finely chop the onion, garlic and saute them in olive oil. Add the finely chopped celery, tomato paste (dissolved in water), spices, sugar and lemon juice. Stir and add 1 cup of water. Boil the sauce for 20 minutes approximately. Place the giant beans in a baking pan and pour the sauce over them. Garnish the beans with the tomato slices; season with a little salt and pepper and bake for 30-40 minutes at 180oC. Serve warm or at room temperature.