A rustic open-faced tart with slow-cooked sweet onions and tangy Gorgonzola on a flaky puff pastry base.
This tart draws on the classic Italian pairing of sweet caramelized onions and pungent blue cheese, a combination found in regional dishes across northern Italy where Gorgonzola is produced. Thinly sliced onions are cooked low and slow until deeply golden and jammy, spread over puff pastry, and dotted with crumbled Gorgonzola before baking until the pastry puffs and turns deep golden. The technique that matters most is genuinely caramelizing the onions -- not just softening them, but cooking them 30-40 minutes over low heat, stirring occasionally, until they turn a deep amber and taste sweet rather than sharp. Rushed onions at high heat will brown unevenly and taste bitter rather than achieving that sweet, jammy quality that makes the tart work. Baked until the pastry is crisp and golden and the cheese has melted into pockets throughout the onions, this tart is simple enough for a weeknight but elegant enough for guests, best served slightly warm so the Gorgonzola stays soft and the pastry stays crisp.
Serves 4
Melt butter with olive oil in a wide skillet over medium-low heat. Add onions and salt, cook, stirring occasionally, 30-40 minutes until deeply golden and jammy.
Stir in balsamic vinegar during the last 2 minutes of cooking, scraping up any browned bits. Remove from heat and cool slightly.
Preheat oven to 200C/400F. Roll out puff pastry on a lined baking sheet and score a border about 2cm from the edge without cutting all the way through.
Spread caramelized onions within the scored border. Dot with crumbled Gorgonzola and scatter thyme leaves.
Brush the pastry border with egg wash. Bake 25-30 minutes until the pastry is puffed and deep golden.
Scatter with walnuts if using, slice, and serve warm.
Cook the onions low and slow for the full 30-40 minutes -- there's no real shortcut to proper caramelization, and rushing it with higher heat gives bitter, unevenly browned results.
Score a border around the pastry edge without cutting through completely; this lets the border puff up into a proper crust while the center stays flat for the filling.
Cool the onions slightly before spreading on the pastry so they don't start melting the butter in the dough prematurely.
Use goat cheese instead of Gorgonzola for a milder, tangier flavor.
Add crispy pancetta or prosciutto for a heartier, non-vegetarian version.
Make individual smaller tarts instead of one large one for easier portioning at a dinner party.
Refrigerate covered up to 3 days. Reheat in a 180C/350F oven for 8-10 minutes to recrisp the pastry; avoid microwaving as it makes the crust soggy.
Caramelized onion and blue cheese tarts reflect a broader European tradition of pairing sweet, slow-cooked alliums with pungent cheeses; Gorgonzola specifically has been produced in Lombardy and Piedmont since the Middle Ages and remains one of Italy's most distinctive blue cheeses.
Yes, caramelized onions keep refrigerated for up to a week -- make a big batch ahead and just assemble and bake the tart when ready.
The pastry was likely not cold enough when it went into the oven, or the oven wasn't fully preheated -- keep pastry chilled until the last moment and always bake in a fully preheated oven.
Yes, a milder blue cheese, goat cheese, or even fontina all work well as substitutes, adjusting the amount to taste since flavors vary in intensity.
Per serving (220g / 7.8 oz) · 4 servings total
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