Soak the beans overnight in a large bowl with enough cold water.
Preheat the oven to 180°C/356° F with the fan on.
Strain the beans and place them in a deep pot. Cover with water and simmer for 40-50 minutes, or until the beans are soft, skimming at regular intervals.
Strain the beans, retaining 200 ml (~1 cup) of the water.
Pour half the olive oil into a small frying pan and heat over medium heat. Add the onion, garlic, red peppers, celery, and carrots. Cook for 5 minutes until they caramelize.
Place the giant beans in a heatproof dish. Add the vegetables, tomatoes, the remaining olive oil, the retained water, dill, and season with salt and pepper on top.
Bake for 50-60 minutes, or until the beans are mushy and well-cooked.
Serve the beans hot, straight from the oven.