1. Prepare a caramel : pour the sugar, the water and the lemon juice into a pan. Heat on a medium heat, without stirring, until the caramel begins to change colour (about 4 to 5 minutes).
2. Split the vanilla pod lengthwise. As soon as the caramel has a good color, remove it from the heat, add the butter and the vanilla seeds, scraping them with the tip of a knife. Stir with a wooden or metal spoon. Without waiting, pour the caramel into the deep lid.
3. Preheat the oven to 200 °C (th. 7).
4. Prepare the tart : peel the pears, cut them into two and remove the cores. Arrange them on the caramel. Place the flaky pastry on top, pushing the edges down at the sides so that the pears are completely covered.
5. Cook for 30 minutes in the lower part of the oven. Then, while still warm, upturn the tart, onto a serving dish.