Shallots Go Glam in This Showstopper Tart

Savory upside-down tarts always impress with their dramatic unveiling, high-gloss finish, and contrast of softened, brightly glazed vegetables atop a flaky crust. Alliums work particularly well thanks to their complex flavor and visually striking cut surfaces.

But what I love most about this tart is how flexible it is: There are multiple components, but each is straightforward and some can be prepared in advance. It’s equally lovely served warm or at room temperature, and it’s one of those all-purpose preparations that shifts seamlessly from appetizer to side dish to main course. (I’ve even snuck a piece for dessert.)

The foundation—and eventual crown—of any good upside-down tart is the layer of tender, deeply caramelized fruit or vegetables. In savory versions, the sugar you add to the pan is often a punchy vinegar syrup that highlights the vegetables’ natural savory sweetness and naps them with a polished sheen, so I reduced some cider vinegar with a little sugar in a 10-inch skillet and ran a few tests with both onions (wedges and rounds) and shallots (halved lengthwise) to see which might look and taste the nicest.

I tightly arranged the raw pieces cut side down in the syrup, blanketed them with a round of commercial puff pastry (which tastes great, is sold widely, and makes the dish easy to execute), and baked the tarts until the pastry was puffed and deeply golden.

The shallot version outshone the onion one in every way, which convinced me that distinctions between their sugar and water contents and pungency, plus their shape and size variations, made a meaningful difference here. But I had an idea for the onions and realized I could take advantage of the alliums’ unique qualities in different ways.

Shallots contain roughly twice as much sugar as onions and about 10 percent less water, and they’re milder because they’re lower in the precursor sulfur compound isoalliin that leads to pungency when the vegetables’ cells are cut open—all of which added up to a nicely sweet-savory tart. And because shallots are smaller, with thinner layers, they baked up softer and I could fit more into the pan.

Still, it was important to precook them to remove moisture that would thwart browning and to ensure that they softened. But rather than brown them in the skillet, which would risk uneven cooking and require maneuvering them in the pan to monitor color, I took advantage of the preheated oven’s uniform heat and roasted the halves (brushed with oil and seasoned with salt and pepper) cut side down on a rimmed baking sheet.

Within about 30 minutes, they were tender and sweet throughout and deeply browned on the cut sides; plus, they’d shrunken enough that I could cram 18 of the elegant teardrop-shaped halves into the skillet for a particularly dazzling presentation. Only their dehydrated outer layers gave me pause, so I roasted them skin-on to insulate the flesh and then plucked off their protective “jackets” as soon as the shallots had cooled.

As for the onions, their pungency had great potential. The conversion of those sharp-tasting compounds into other flavorful sulfur-based molecules such as sweet zwiebelanes and savory 3-mercapto-2-methylpentan-1-ol (MMP) is what makes them so complex when they’re caramelized. And a heap of jammy onions could anchor the shallots to the pastry, add even more depth, and give the tart nice stature. 

While the shallots roasted, I caramelized a pound of thin-sliced onions per our unique quick method: Steam them in a covered pan with water (which encourages them to rapidly shed their liquid and collapse) and a pinch of baking soda (which raises their pH and encourages flavorful Maillard browning), and then stir them uncovered until they concentrate and brown deeply.

While the shallots and onions cooled, I made the vinegar syrup, enriching it with a little butter for body and shine, and arranged the roasted, peeled shallot halves in the syrup in a snug circular pattern. I spread the caramelized onions over the top and covered the pan with a round of pastry. I baked the whole thing until the crust puffed and browned deeply. 

After letting the hot pan cool, I carefully inverted the tart onto a plate and let it cool for 15 minutes more before cutting a wedge so that the components had time to set up a bit. The contrast between the sweet-tart syrup, tender glazed shallots, jammy onions, and crisp pastry was a knockout. But I took it up one more notch with a creamy dollop—Boursin (the shallot-chive version) whipped with a little sour cream—for my own little allium hat trick.