Thinly slice the onions. Heat the olive oil in a large pan, add the onions and a good pinch of salt and cook over medium-low heat for 25–30 minutes, stirring occasionally, until very soft, golden and jammy. Add the balsamic and cook for another 1–2 minutes. Let cool slightly.
Preheat the oven to 200°C conventional / 180°C fan.
Place the shortcrust pastry on a lined baking tray, score a small border around the edge and prick the centre with a fork.
Spread the caramelized onions evenly over the pastry. Slice or quarter the fresh figs and arrange them on top, then crumble over the goat cheese and finish with black pepper.
Bake for 25–30 minutes, until the pastry is golden and crisp.
Meanwhile, roughly chop the walnuts and toast them in a dry pan for 2–3 minutes.
Finish the warm tart with the toasted walnuts, a little finely chopped fresh rosemary and a light drizzle of honey.
