Rustic Onion Tart

Rustic onion tart with caramelized red onions, Gruyere cheese, and a butter crust.

Rustic Onion Tart
Elise Bauer

Is there nothing better than caramelized onions?

Yes, they take some time to make, but they're not hard to cook, and it's so so worth the effort.

Here is great way to enjoy them—in a simple rustic tart, with some shredded Gruyere cheese and a sprinkling of thyme.

You can either use a ready made frozen crust (the flat or folded kind), or make your own crust.

Rustic Onion Tart

Prep Time 20 mins
Cook Time 100 mins
Total Time 2 hrs
Servings 4 servings

1 packaged, flat store-bought pie crust will work if you don't want to make the pastry dough from scratch.


For the sour cream pastry dough:

  • 1 1/4 cup (160g) all-purpose flour

  • 1 teaspoon kosher salt

  • 5 ounces unsalted butter (1 1/4 sticks, 10 tablespoons, 137g), cubed

  • 1/4 cup sour cream

For the filling:

  • 3 medium red and/or yellow onions (enough for 5 to 8 cups, sliced)

  • 2 tablespoons extra virgin olive oil

  • 1 tablespoon unsalted butter

  • Kosher salt

  • 1 teaspoon balsamic vinegar

  • 3/4 cup (not packed) roughly grated Gruyère cheese

  • 1/2 teaspoon fresh thyme, chopped, or 1/4 teaspoon dried thyme


  1. Prepare the dough:

    To prepare the dough, whisk the flour and salt together in a large bowl. Cut the butter into the flour with your hands or with a pastry cutter, until the mixture resembles a coarse meal.

    Add the sour cream and use a fork to incorporate it into the dough. Gather the dough into a ball and shape it into a disk. Wrap it with plastic wrap and chill for at least an hour.

  2. Prepare the onions:

    Peel and slice the onlions lengthwise from stem to root. (See How to Slice an Onion)

  3. Caramelize the onions:

    Heat olive oil and butter in a large, heavy-bottomed sauté pan on medium heat. Once the butter has melted, add the onions and sprinkle a little salt over them.

    Cook, stirring occasionally for 10 minutes, until the onions have softened and are translucent.

    Elise Bauer
    Elise Bauer

    Reduce the heat to medium low and cook for an additional 40 minutes, stirring occasionally, until the onions are well browned.

    Add balsamic vinegar and cook for 10 minutes more, until onions are completely caramelized. Remove from heat.

  4. Preheat the oven to 450°F.
  5. Roll out the dough:

    Remove the dough from the refrigerator and let stand for 5 minutes before rolling it out.

    Roll out the dough on a lightly floured surface to a 13-inch diameter. Transfer the crust dough to a cookie sheet lined with parchment paper.

  6. Layer the cheese and onions onto the dough:

    Place all but a couple tablespoons of the cheese in the center of the dough. Spread to within 1 1/2 inches from the edges. Add the caramelized onions, layering them on top of the cheese. Sprinkle with thyme.

    Elise Bauer
    Elise Bauer
  7. Pleat the edges:

    Fold the edges of the crust dough over so that a small circle of onion is still showing in the centre of the tart. Sprinkle the remaining cheese over the top of the tart.

    Elise Bauer
    Elise Bauer
  8. Bake:

    Place in the oven on the middle rack. Bake for 10 minutes at 450°F. Reduce the heat to 350°F and bake for an additional 20 to 25 minutes, until the crust is golden brown.

    Remove from oven and let sit for 10 minutes before serving.

Nutrition Facts (per serving)
633 Calories
49g Fat
39g Carbs
12g Protein
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Nutrition Facts
Servings: 4
Amount per serving
Calories 633
% Daily Value*
Total Fat 49g 62%
Saturated Fat 26g 131%
Cholesterol 116mg 39%
Sodium 793mg 34%
Total Carbohydrate 39g 14%
Dietary Fiber 2g 7%
Total Sugars 4g
Protein 12g
Vitamin C 4mg 19%
Calcium 262mg 20%
Iron 2mg 12%
Potassium 207mg 4%
*The % Daily Value (DV) tells you how much a nutrient in a food serving contributes to a daily diet. 2,000 calories a day is used for general nutrition advice.
Nutrition information is calculated using an ingredient database and should be considered an estimate. In cases where multiple ingredient alternatives are given, the first listed is calculated for nutrition. Garnishes and optional ingredients are not included.