Drain and rinse the soaked giant beans thoroughly. Place them in a large pot of boiling water over high heat. Cook for 1 hour and 30 minutes, until the beans are almost tender but still hold their shape.
While the giant beans are boiling, skim off any foam with a spoon. Once the foaming stops, reduce the heat to a simmer.
Drain the beans again, reserving some of the cooking water, and transfer them to a 27 x 20 cm baking dish.
Preheat the oven to 180°C (356°F), fan mode.
In a pan over medium heat, add 3 tablespoons of the olive oil and sauté the onions and Florina peppers until softened.
Stir in the tomato paste, smoked paprika, and oregano, cooking briefly to release the flavors.
Add the sliced sausages and canned tomatoes, season with salt and freshly ground black pepper, and stir to combine.
Pour the sauce over the beans in the baking dish and mix gently to coat them evenly.
Chop the celery finely and sprinkle it over the beans. Pour in the reserved bean cooking water along with the remaining olive oil, season again with salt and pepper, and stir lightly.
Cover the baking dish with aluminum foil and bake for 60–90 minutes, until the beans are fully tender and the sauce is rich and flavorful.
Serve the dish hot, straight from the oven.