1. Heat olive oil in a pan. Sauté onion and celery with a pinch of salt for 5 minutes over medium heat.
2. Add walnuts, apple, honey, and water. Cook for another 4 minutes.
3. Preheat oven to 180°C.
4. Spread filling onto 4 slices of bread, top with remaining slices,
and press the edges to seal. Brush with olive oil and bake until golden.