Dinner · Vegetarian · Fall · Baking

Caramelized Onion and Gruyere Tart

Name: Caramelized Onion and Gruyere Tart
Prep time: 15 min
Cook time: 65 min
Total time: 85 min
Yield: 6 tart portions
Ingredients: 850 g yellow cooking onions, thinly sliced; 3 tbsp olive oil, plus a little for the baking sheet; ½ tsp fine salt, or to taste; freshly ground black pepper, to taste; 1 ready-rolled all-butter puff pastry sheet, about 320 g, thawed according to package directions; 1 large egg, beaten; 2 tbsp Dijon mustard; 25 g fine plain breadcrumbs; 80 g Gruyère cheese, coarsely grated; 1 tsp fresh thyme leaves, plus a little for serving

Slow-cooked onions and tangy mustard fill a flaky puff pastry tart, finished with Gruyère and fresh thyme. Let the onions cool briefly before filling the pastry.

Edited by Tasty Recipes editorial team

Published 2026-09-20

1 hr 25 minMakes 6 tart portions

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Rectangular puff pastry tart with a raised golden border, caramelized onions, melted Gruyère and thyme, with one square slice lifted

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How to make Caramelized Onion and Gruyere Tart

Timing breakdown · 3 timed stages

Allow about 15 minutes to slice onions and prepare ingredients, roughly 40 minutes to soften and brown the onions, 20 to 25 minutes to bake and 5 minutes to cool before slicing. Onion browning depends on pan and heat.

Preparation: 15 min
Cooking: 65 min

Cooking Step 1: 30 minutes
Baking Step 5: 20 to 25 minutes
Baking Step 5: 5 minutes

Durations are taken from the method and may overlap. Some preparation steps are not timed. Optional stages are separate from the main meal.

01
Warm the olive oil in a large lidded skillet over low heat. Add the onions and salt, stir to coat and cover. Cook for about 30 minutes, stirring every few minutes, until the onions are very soft; lower the heat if they catch on the pan.
02
Remove the lid and cook for about 10 more minutes over medium-low heat, stirring often, until the onions are deep golden and most moisture has evaporated. Season with black pepper and spread on a plate to cool briefly. Do not add hot onions to the pastry.
03
Heat the oven to 180°C (350°F), or 160°C (325°F) for a fan oven. Line a baking sheet with parchment and lightly oil any exposed surface. Unroll the chilled pastry onto the sheet. With a sharp knife, score a 2 cm border inside the edges without cutting through. Brush the border with the beaten egg, keeping the egg off the cut sides.
04
Spread the Dijon mustard inside the scored border, then scatter over the breadcrumbs to absorb onion moisture. Spread the cooled onions evenly inside the border. Scatter the grated Gruyère and thyme over the onions, leaving the border clear.
05
Bake for 20 to 25 minutes, until the pastry border is puffed and golden, the base looks baked rather than damp, and the cheese has melted. If the base needs more time, continue baking in short intervals and check it again. Transfer to a rack for 5 minutes, then cut into six portions and add a little fresh thyme.
06
Cool any leftovers promptly and refrigerate in a covered container within 2 hours. Use within 2 to 3 days. Reheat on a baking sheet in a moderate oven until hot and the pastry crisps again; avoid storing the tart at room temperature overnight.

Recipe questions

Can I cook the onions ahead?

Yes. Cook them until the excess liquid evaporates, cool promptly and refrigerate in a covered container. Bring them toward room temperature before spreading over chilled pastry.

Why are breadcrumbs under the onions?

They help catch extra onion moisture so the pastry base can bake through. Keep the onions inside the scored border and do not overload the tart.

Can I use Parmesan instead of Gruyère?

The credited source uses Parmesan. Gruyère melts differently and gives this version a softer topping; the original cheese remains an option if you prefer the source version.

How should I check the pastry?

Look for a visibly puffed golden border and lift a corner carefully with a spatula to check that the underside is cooked. Oven and pastry brands vary, so use those cues as well as the clock.

What goes with the tart?

Cut it into six smaller portions for a light meal with a green salad, or serve smaller squares alongside other dishes. The recipe does not include a salad in its ingredients or timing.

Cooking & storage

Use the recipe’s specific storage directions when provided. Read the food storage guide for chilling and reheating food, and the measures guide before changing portions or ingredients.

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Adapted from Thane Prince at Good Food. The source method of slowly caramelizing onions, scoring puff pastry and layering Dijon with breadcrumbs is retained; this version replaces the Parmesan topping with Gruyère and specifies cooling and pastry-doneness cues.

Adapted from Thane Prince at Good Food.

Additional guidance: Foodland Ontario: onion availability · Health Canada food storage guidance