Steps
1. Preheat the oven to 180°C (350°F).
2. In a saucepan, melt the butter over medium heat. Add the sliced onions and cook until caramelized, stirring occasionally.
3. In a separate bowl, combine the strong coffee, honey, and milk. Mix well.
4. Roll out the pastry dough and transfer it to a tart pan.
5. Spread the caramelized onions evenly over the pastry.
6. Pour the coffee-honey mixture over the onions.
7. Bake in the preheated oven for 25-30 minutes, or until the tart is golden brown and set.
8. Allow the tart to cool slightly before serving.