Caramelized onion tarte tatin

Method

1. Peel the onions and halve each one through the root, so the root end holds the two halves together. Melt the butter in a 25 cm ovenproof skillet over low to medium heat. Arrange the onions cut-side down in the pan in a single snug layer, tuck the thyme sprigs among them, and season with the salt and pepper.
2. Cook low and slow, mostly undisturbed, for 30 to 40 minutes. The onions should turn deep golden brown and completely tender, with the cut sides caramelizing against the pan while the tops soften and collapse slightly. Check every 10 minutes or so; if one side is browning much faster than another, rotate the pan rather than stirring, and carefully turn each onion half once, partway through, so both cut faces get a chance to caramelize. Resist the urge to raise the heat: onions that colour too fast will scorch on the outside while staying raw at the core.
3. Once the onions are deeply caramelized and a skewer slides through with no resistance, add the vinegar and brown sugar. Let it bubble and reduce for 1 to 2 minutes, swirling the pan gently, until the liquid turns syrupy and coats the onions in a glossy, deep amber glaze.
4. Take the pan off the heat. Nudge the onions into one even, cut-side-down layer, packing them close since they will shrink further in the oven. Heat the oven to 200°C (400°F).
5. Lay the cold puff pastry over the pan, prick it all over with a fork, and tuck the overhang down inside the rim of the pan rather than folding it over the top, so it forms a snug lid once baked.
6. Bake for 25 to 30 minutes, until the pastry is well risen and a deep, even golden brown, checking after 20 minutes and rotating the pan if it is browning unevenly.
7. Rest the tart in the pan for 5 minutes. Run a thin knife around the inside edge of the pan to loosen it, then place a large plate over the top and, holding both firmly together with oven mitts, invert the pan in one confident motion. Lift the pan away; if any onions stick, lift them off with a spatula and set them back in their gap.
8. While the tart is still warm, crumble the goat cheese over the top and scatter with extra thyme leaves. Slice into wedges and serve.

Notes

• This is best eaten warm or at room temperature the day it’s made, while the pastry is still crisp. Leftovers keep, covered, in the fridge for up to 2 days; reheat wedges in a 180°C oven or a hot dry pan for 8 to 10 minutes so the pastry recrisps, since a microwave will leave it soft and soggy.
• The single biggest risk with this dish is rushing the onions. Deep caramelization at low heat takes the full 30 to 40 minutes; turning the heat up to save time browns the outsides while the centres stay firm and raw, and the glaze never develops properly. Patience here is what makes the finished tart taste sweet and rounded rather than sharp and raw.
• Small, roughly golf-ball-sized onions or banana shallots work best because they hold their shape through the long cook and flip out as neat, distinct wedges. Very large onions can be used but should be cut into thick wedges instead of halves, since a whole large onion half won’t cook through evenly in the same time.
• A splash of sherry vinegar gives a savory, slightly nutty edge; balsamic leans sweeter and more syrupy. Either works, so use whichever you have.

Ingredient substitutions

• All-butter puff pastry is the traditional and preferred choice here: it browns more deeply, tastes richer, and has more distinct layers than pastry made with vegetable shortening. In Germany, refrigerated Blätterteig from the chilled section of the supermarket is usually all-butter already and is a reliable default. In the US and Canada, most supermarket puff pastry, including common brands like Pepperidge Farm, is made with vegetable shortening, so look specifically for an all-butter brand such as Dufour if you want the same richness.
• Homemade rough puff pastry is a reasonable middle ground if you’d rather make your own without committing to full classic lamination. Since the pastry ends up as a flat lid rather than a laminated showpiece, the extra effort of true puff pastry buys little visible benefit here.
• Whichever pastry you use, it must go onto the pan cold, straight from the fridge. Pastry that has softened at room temperature won’t puff properly in the oven and is more likely to shrink or slide as it bakes.

Sources consulted

• The Kitchn
• Jamie Oliver