These upside down onion tarts are an irresistible blend of sweet caramelized onions and nutty Gruyere, all nestled under golden, flaky puff pastry. A snack or appetizer worth every bite.
Perfectly balanced between savory, sweet, and spicy, upside down onion tarts bring elegance to the table with minimal effort. They’re ideal for gatherings, brunches, or cozy nights in.
Star Players in These Upside Down Onion Tarts
- Puff pastry – The buttery, flaky foundation that puffs beautifully to encase the savory filling underneath.
- Sweet onion – Thinly sliced for quick caramelization, offering natural sweetness and deep flavor.
- Honey – Adds a glossy finish and balances the savory elements with mellow sweetness.
- Balsamic vinegar – Brings acidity and richness, intensifying the onion’s caramel notes.
- Chili flakes – A subtle kick that offsets the sweetness and adds depth.
- Gruyere cheese – Melts into creamy layers with nutty, slightly salty undertones.
- Fresh thyme – Earthy and aromatic, complementing the onions and cheese with herbaceous warmth.
- Egg wash – Ensures the pastry gets that irresistible golden, shiny crust.
How to Make Them: Upside Down Onion Tarts, Step by Step
Start by preheating your oven to 400°F. While that’s heating up, slice your onions thin and cut your thawed puff pastry into even rectangles.
On a parchment-lined baking sheet, spoon small puddles of honey and balsamic vinegar, sprinkle on some chili flakes, and tuck in a sprig of thyme.
Top each section with a round of sliced onion and a generous layer of Gruyere cheese. Then cover with the puff pastry, tucking in the edges if needed, and brush the tops with beaten egg.
Bake until the tops are golden brown and the bottoms have transformed into sweet, savory caramelized bliss — about 20 to 25 minutes.
Pro Tips & Flavor Twists for Upside Down Onion Tarts
Make sure your puff pastry is fully thawed before you begin — this makes cutting and layering far easier and ensures even puffing in the oven.
For an extra savory punch, sprinkle a touch of mushroom or beef bouillon powder over the onions before layering on the cheese and pastry. You can also experiment with different cheeses like Fontina or aged cheddar for a twist on the classic Gruyere.
Love heat? Add a touch more chili flake or even a few slices of jalapeño beneath the pastry for a spicy kick.
Keeping Them Crispy: How to Store Upside Down Onion Tarts
Let the tarts cool completely before storing to prevent sogginess. Keep them in an airtight container in the fridge for up to three days.
To reheat, use a toaster oven or conventional oven at 350°F until warmed through and crisp again — around 10 minutes. Avoid microwaving, as it softens the pastry.
For longer storage, freeze on a tray until solid, then transfer to a freezer-safe bag. Reheat from frozen in a hot oven until crisp and bubbly.
Upside Down Onion Tarts
Ingredients
- 1 tablespoon balsamic vinegar
- 3 tablespoons honey
- 1 sheet puff pastry thawed
- Fresh thyme leaves salt, and black pepper, to taste
- 1 teaspoon chili flakes
- 3 sweet onions thinly sliced
- 1 egg beaten
- 18 slices Gruyere cheese thinly sliced (or as needed)
Instructions
- Begin by preheating the oven to 400°F. Prepare the puff pastry by slicing it into rectangular portions while simultaneously organizing the onions and cheese.
- Line a baking tray with parchment paper, and in separate sections of the tray, add a drizzle of honey, a dash of balsamic vinegar, a sprinkle of chili flakes, and a few thyme leaves.
- Carefully place a slice of onion atop each mixture, followed by layering with slices of Gruyere cheese.
- Cover each assembly with a rectangle of puff pastry and brush the tops with the beaten egg.
- Transfer the tray to the oven and bake for 20 to 25 minutes, or until the pastry is golden and the onions are thoroughly caramelized.
