Caramelized Onion and Gruyère French Onion Tart

By AislePrompt Editorial · Published 2026-06-08 · Updated 2026-06-08

How this recipe was made — this recipe was drafted by AislePrompt's AI-assisted pipeline from technique templates and USDA-sourced nutrition data, and published by AislePrompt Editorial — see how AislePrompt recipes are developed and reviewed.

A savory tart featuring slowly caramelized onions and nutty Gruyère cheese baked in a buttery pastry crust. This French-inspired brunch recipe, ready in about 55 minutes, layers puff pastry sheet, yellow onions, unsalted butter into a dessert worth slowing down for — great for weekend baking, holiday tables, or any time you want a sweet payoff at the end of a meal. Each serving lands at about 350 calories and feeds 6, so it slots into a weekend bake or a special-occasion dessert tray. Tap "Plan a meal with the AI" below to drop this recipe into your week and send the ingredients straight to Instacart. Nutrition values are approximate, calculated from USDA FoodData Central (fdc.nal.usda.gov) — verify against ingredient labels for any health-driven dietary plan.

Prep: 15 min Cook: 40 min Serves 6 French cuisine 350 cal/serving

Build a week of meals around this recipe → Shop this recipe on Instacart → Shop equipment ↓

Ingredients

1 sheet (thawed if frozen) puff pastry sheet
3 medium, thinly sliced yellow onions
3 tbsp unsalted butter
1 tbsp olive oil
1 tsp fresh thyme leaves
1/2 tsp salt
1/4 tsp black pepper
1 cup shredded Gruyère cheese
1 (for egg wash) egg yolk

Sheet pans that don’t warp in a hot oven → Best Sheet Pans in 2026: Top Picks + Buying Guide

Affiliate disclosure: AislePrompt may earn a commission on qualifying purchases made through Amazon links, at no additional cost to you. See our policy →

Equipment for this recipe

Tools this recipe's steps call for, with a link to each on Amazon.

More Bakeware & baking tools → Shop all kitchen tools →

Instructions

Step 1: Heat 3 tbsp unsalted butter and 1 tbsp olive oil in a large skillet over medium-low heat. Add 3 thinly sliced medium yellow onions, 1 tsp fresh thyme leaves, 1/2 tsp salt, and 1/4 tsp black pepper. Cook, stirring occasionally, for 30-35 minutes until the onions are deeply golden and caramelized.
Step 2: Preheat oven to 400°F. Roll out 1 sheet puff pastry on a baking sheet lined with parchment paper. Score a 1-inch border around the edges without cutting through.
Step 3: Spread the caramelized onions evenly within the scored border. Sprinkle 1 cup shredded Gruyère cheese evenly over the onions.
Step 4: Brush the 1 egg yolk lightly beaten over the pastry border to create a golden crust.
Step 5: Bake the tart for 20-25 minutes until the pastry is puffed and golden and the cheese is melted and bubbly. Remove from oven and let cool 5 minutes before slicing and serving.