By Yotam Ottolenghi and Eric Asimov, The New York Times

LONDON — I am routinely asked to explain how a certain dish came about: What was the process that made me combine this particular set of ingredients in that particular way? The truth is, more often than not, I don’t have an answer. I simply don’t know.

The temptation in such cases is to try and assign some kind of internal logic to the events — something in the manner of “I needed some acidity there, and a bit more texture, so I added a spoonful of this and a sprinkling of that.” And although it makes sense to see this creative process as something sound and structured, I suspect that often, rather than looking for a solution to make a dish better, more balanced, etc., there is something much more erratic that is going on. It is the ingredients that are in control, not I.

This potato, Swiss chard and sumac onion gratin is a good example. As far as dishes go, it’s hard to find one less in need of fixing or rebalancing than the old gratin dauphinois. The French nailed the potato gratin right from the word “go,” taking three ingredients — potatoes, butter and milk or cream (plus some basic seasonings) — and reaching creamy-starchy flawlessness. There is really no reason to mess about with this one.

Call in Swiss chard, an age-old disrupter. Looking at dishes where I’ve used chard in the past, it’s hard to ignore a pattern: if cheese, cream or yogurt are involved, you are likely to find chard in some shape or form; a bulky, carby mass — ditto. Chard stalks and leaves, available year-round, tend to just come up naturally every time I am about to cook something rich and dairy-laden or something hearty and bulky.

What makes them so perfect for such contexts are their naturally sharp, almost metallic notes, and an echo of the soil, similar to what you get with beets, which are members of the same family. While planning to try a Comté pie with crispy polenta crust about a decade ago — a recipe I adore to this day — I must have had chard shouting out my name, saying, “without me, things are just going to be too rich.”

That extraordinary ability to cut through the unctuous uniformity of a béchamel or a pork pâté or pie made chard a particularly valuable green in provincial French cooking. Much earlier, though, it was adopted by the Arabs and spread across the Mediterranean. Delicacies such as a Spanish dish of chard with raisins and pine nuts, or pastries stuffed with a dense chard filling common in Turkey, Greece and the Balkans are remnants of that influence and further examples of how effective chard is in keeping its integrity and flavor through cooking. Though spinach works well in many of these dishes, and is often cited as substitute to chard, it hasn’t got the flavor or the body to quite match the chardy impact.

With such a long history — both globally and in my modest kitchen — it is no wonder that Swiss chard came to mind when it was time to have another look at the age-old, creamy potato gratin. Paired with sumac, a spice with intense tartness and, like chard, surefire appeal, the French classic hasn’t been improved by chard, really, but rather transformed into a whole different kettle of fish (or rather, potato). Yes, the starchy, velvety texture is still there, as is the adorable whiff of butter and cream, but the whole thing has been turned on its head with an earthy, sharp, fresh punch that is as seductive as the original.

And what did I have to do with all this? Less than you’d imagine. It’s the Swiss chard that’s been calling the shots.

Potato Gratin With Swiss Chard and Sumac Onions

Yield: 6 to 8 servings

Total time: 2 1/2 hours

Ingredients

For the Sumac Onions:

  • 1/4 cup (60 milliliters) olive oil
  • 1 1/2 pounds (680 grams) red onions, peeled, halved and sliced about 1-centimeter thick
  • Kosher salt
  • 1 1/2 tablespoons dried sumac

For the gratin:

  • 2 1/2 pounds (1.1 kilograms) Yukon Gold potatoes (skin-on), sliced using a mandoline or very thinly sliced into rounds
  • 1 3/4 pounds (800 grams) green Swiss chard, leaves torn from stems (stems reserved for another use) and roughly shredded (about 6 packed cups/315 grams)
  • 2/3 cup (30 grams) roughly chopped fresh parsley, plus 2 tablespoons (5 grams) finely chopped parsley for garnish
  • 6 garlic cloves, minced
  • 2 teaspoons finely grated lemon zest, plus 1/4 cup juice (from 2 lemons)
  • Kosher salt and freshly ground black pepper
  • 8 ounces (225 grams) mature cheddar, roughly grated (about 3 cups)
  • 1 cup (240 milliliters) chicken or vegetable stock
  • 3 tablespoons heavy cream (double cream)

For the Brown-Butter pine nuts:

  • 2 tablespoons (30 grams) unsalted butter
  • 1/2 cup (65 grams) pine nuts
  • Kosher salt

Directions

1. Heat oven to 375 degrees Fahrenheit (180 degrees Celsius).

2. Prepare the onions: Add the oil to a large, ovenproof lidded skillet and heat over medium-high. Add the onions and 1 teaspoon salt to the hot oil and cook, stirring occasionally, until softened and browned, 15 to 20 minutes. Stir in the sumac and remove from the heat. Transfer the mixture to a bowl.

3. Prepare the gratin: Add the potatoes, shredded chard, parsley, garlic, lemon zest, 1 tablespoon salt and 1 teaspoon pepper to a large bowl and mix well to combine. Fold through three-quarters of the sumac onions and half the cheese, then transfer everything to the skillet, smoothing out the top to even out the potato slices. In a measuring cup or bowl, combine the stock, cream and lemon juice, and pour this all over the potato mixture. Cover tightly with foil, then top with the lid. Bake for 1 hour.