Slow-Caramelised Onion Relish with Balsamic and Thyme

By Sprig Originals ✓ · cooked up in Sprig

Three pounds of yellow onions cooked down for well over an hour until they collapse into a dark, jammy, almost meaty relish, then sharpened with balsamic and cider vinegar and perfumed with thyme. It's the jar you want in the fridge door: spooned onto burgers and steak sandwiches, folded into a cheese board, stirred into gravy, or spread under melted Gruyère on toast. The only real technique here is patience and low heat — rush it and you get sweet fried onions, take your time and you get preserve-shelf depth.

Ingredients

• 3 lb yellow onions (about 5 large), halved and sliced 1/8-inch thick
• 2 tbsp extra-virgin olive oil
• 2 tbsp unsalted butter
• 1 1/4 tsp kosher salt, plus more to taste

• 3 tbsp dark brown sugar, packed
• 1/4 cup balsamic vinegar
• 3 tbsp apple cider vinegar
• 2 tbsp water, plus more as needed
• 6 sprigs fresh thyme
• 1 bay leaf
• 3/4 tsp freshly cracked black pepper
• 1/4 tsp crushed red pepper flakes (optional)
• 1 tsp whole-grain or Dijon mustard

Instructions

1. Trim the root and stem ends from the onions, halve them pole to pole, peel, and slice into thin 1/8-inch half-moons. Consistency matters more than speed — evenly cut onions collapse at the same rate. You should have about 10 packed cups; the pile will look absurd and will cook down to roughly 2 cups.
2. Set a wide 12-inch skillet or 5-quart Dutch oven over medium heat and add the olive oil and butter. When the butter foams, add the onions in two additions, stirring the first batch down for a minute to make room for the second. Sprinkle over the 1 1/4 teaspoons salt.
3. Cook over medium heat for 12 to 15 minutes, stirring every couple of minutes, until the onions have wilted, released their liquid, and dropped to about half their original volume. They should still be pale.
4. Turn the heat down to medium-low (you want a lazy, quiet sizzle, not a crackle) and settle in for the long part: cook 55 to 70 minutes, stirring every 5 to 8 minutes and scraping the bottom and corners of the pan. The onions will go from blond to gold to a deep russet brown and shrink to a glossy tangle. If the fond on the bottom of the pan ever looks like it's tipping from brown to black, add 1 to 2 tablespoons water and scrape it up immediately — that scraped fond is where the flavour lives.
5. When the onions are uniformly mahogany, sweet, and jammy enough to hold a trail when you drag the spoon through them, stir in the brown sugar, thyme sprigs, bay leaf, black pepper, and red pepper flakes if using. Cook 3 minutes, stirring constantly, until the sugar has melted and coated everything.
6. Pour in the balsamic vinegar, cider vinegar, and 2 tablespoons water. Raise the heat to medium and scrape hard as it bubbles — the vinegar will loosen every bit of stuck browning. Simmer 8 to 12 minutes, stirring often, until the liquid has reduced to a syrup that coats the onions and no thin liquid pools when you pull the spoon across the pan bottom.
7. Off the heat, fish out the bay leaf and the bare thyme stems (the leaves will have fallen off into the relish). Stir in the mustard. Taste and adjust: another pinch of salt for depth, a teaspoon of cider vinegar if it reads too sweet, a little more black pepper for bite. It should taste sweet-sour-savoury in roughly equal measure.
8. Let the relish cool in the pan for 20 minutes, then pack into two clean 8-ounce jars, pressing out air pockets with the back of a spoon. Cool completely before lidding and refrigerating. The flavour tightens and deepens overnight, so make it a day ahead if you can.

Chef's tips

Nutrition (per serving, estimated)

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