Polina says: “The French turned what essentially started out as rustic peasant food into a gloriously rich, deeply savoury, and crave-worthy dish. Caramelised onions are turned into a flavourful, velvety-thick broth, which is then ladled into a bowl and topped with a bread slice and a generous layer of nutty, slightly sweet gruyère.

“A grill in the oven results in a bubbling and browned cap of melted cheese that you must fight through to reach the delicious goodness below it. If that’s not a stroke of genius on the French’s part, I don’t know what is!

“If you have homemade beef stock on hand, this is the time to use it. If not, seek out the best variety you can find or substitute chicken stock. I like to use a blend of yellow and red onions along with shallots as the base to lend complexity.”

Serves 4-6

Ingredients

55g unsalted butter
1.4 kg mixed onions (such as yellow, red, and shallots), cut into 6 mm slices
½ tsp flaky sea salt, plus more as needed
½ tsp sugar
3 garlic cloves, 2 cut into thin slices and 1 halved
120ml dry white wine
3 tbsp plain flour
960ml store-bought or homemade beef stock or broth
2 large thyme sprigs
1 large fresh or dried bay leaf
freshly ground black pepper
1 tbsp cognac or dry sherry (optional)
4 to 6 baguette slices
115 to 170g gruyère, coarsely grated

Method

In a large, heavy-bottomed pot, melt the butter over medium heat. Add the onions and stir until they’re fully coated in butter. Cook, stirring occasionally, until the onions have softened and turned translucent, about 20 minutes.

Add the salt and sugar, increase the heat to medium-high, and cook, stirring frequently, until the onions have browned and are caramelised, another 25 to 30 minutes. While you stir, make sure to scrape up the browned bits from the bottom so that they don’t burn.

Lower the heat to medium-low and add the sliced garlic. Cook for another 1 to 2 minutes. Pour in the wine and deglaze the pan, stirring to scrape up the browned bits from the bottom of the pot. Cook until the liquid has evaporated and the onions begin to dry out again, 3 to 4 minutes.

Stir in the flour and cook for another 3 minutes – the mixture will turn into a thick paste. Pour in 240ml of the stock and stir to fully incorporate. Add the thyme, bay leaf and remaining 720ml of stock. Bring to a boil, then lower the heat and simmer for 30 minutes, until the broth thickens.

Remove the bay leaf and thyme sprigs and season with salt and pepper. Add the cognac (if using), and continue to simmer. The soup can be stored in an airtight container in the refrigerator for up to 4 days.

While the soup is simmering, rub each baguette slice with the halved garlic clove and then toast the bread.

About 10 minutes before you’re ready to serve, turn the grill on high. Put four to six ovenproof bowls or large ramekins on a rimmed baking sheet. Ladle the soup into the bowls and top each with a baguette slice. Divide the cheese among the servings, covering both the toast and the soup.

Carefully transfer to the oven and grill until the cheese has melted and begins to bubble and brown, 4 to 8 minutes. Serve immediately.

Alternatively, you can place the garlic-rubbed toasts in an even layer on a baking sheet, top with some of the cheese, and grill until the cheese has melted, 2 to 4 minutes. Top each bowl with a cheesy toast and sprinkle any remaining cheese on top.

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