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Onion Tart with Mustard and Fennel

This savoury onion tart with mustard and fennel is a sophisticated vegetarian dish that celebrates the deep, natural sweetness of slowly caramelised onions. By gently cooking the onions under parchment for over an hour, they transform into a rich, golden topping that pairs beautifully with the aromatic crunch of toasted fennel seeds. The addition of sharp Dijon mustard and salty Parmigiano-Reggiano adds a wonderful depth of flavour to the soft, homemade yeast-leavened crust.

Perfect as a centrepiece for a weekend brunch or sliced into diamonds for an elegant party snack, this tart is incredibly versatile. It can be prepared in advance and served either warm from the oven or at room temperature, making it an excellent choice for alfresco dining or a refined picnic. The combination of simple pantry staples and patient cooking results in a truly comforting homemade meal.

Continue reading below

Ingredients for Onion Tart with Mustard and Fennel

  • 2 1/4 teaspoons active dry yeast (a 1/110g package)

  • 120ml warm water (105-46°C)

  • 350 to 425ml plain flour

  • 1 large egg

  • 80ml plus 1 tablespoon extra-virgin olive oil, divided

  • 2 1/2 teaspoons salt, divided

  • 2 teaspoons fennel seeds

  • 1.4kg yellow onions, halved and thinly sliced

  • 1 tablespoon Dijon mustard

  • 120ml grated Parmigiano-Reggiano

How to make Onion Tart with Mustard and Fennel

Stir together yeast and warm water in a small bowl and let stand until foamy, about 5 minutes. (If mixture doesn't foam, start over with new yeast.)

Put 190g flour in a medium bowl, then make a well in centre of flour and add yeast mixture to well. Stir together egg, 1 tablespoon oil, and 1 1/2 teaspoons salt with a fork. Add egg mixture to yeast mixture and mix with a wooden spoon or your fingertips, gradually incorporating flour, until a soft dough forms. Transfer dough to a floured surface and knead, working in additional flour (up to 30g ) as necessary, until smooth and elastic, about 5 minutes. Transfer dough to an oiled bowl and turn to coat. Cover with plastic wrap and let rise in a draft-free place at warm room temperature until doubled, 1 to 1 1/2 hours.

While dough rises, heat remaining 80ml oil in a 12-inch heavy skillet over medium-high heat until it shimmers, then sauté fennel seeds until a shade darker, about 30 seconds. Stir in onions, remaining teaspoon salt, and 1/2 teaspoons pepper, then reduce heat to medium-low and cover onions directly with a round of parchment paper. Cook, stirring occasionally, until onions are very tender and golden brown, 1 to 1 1/4 hours.

Preheat oven to 191°C with rack in middle.

Knead dough gently on a floured surface with floured hands to deflate. Pat out dough on a large heavy baking sheet (preferably blue steel) into a 15- by 12-inch rectangle, turning up or crimping edge, then brush mustard evenly over dough, leaving a 1/2-inch border around edge. Spread onions evenly over mustard, then sprinkle evenly with cheese.

Bake tart until crust is golden brown, 30 to 35 minutes. Cut into 2-inch squares or diamonds and serve warm or at room temperature.

Disclaimer

While every effort has been made to ensure the information is accurate and up to date, individual needs may vary and dietary requirements can differ based on personal health conditions. Always check food labels and allergen information before preparing or consuming any recipe. If you have specific health concerns, allergies, intolerances, or are following a medically prescribed diet, seek advice from your GP, pharmacist, or a registered dietitian before making significant changes to your diet or lifestyle.

Article history

The information on this page is peer reviewed by qualified clinicians.

  • 16 Jan 2026 | Originally published

    Authored by:

    UK recipe editors

    Peer reviewed by

    UK recipe editors
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