Add olive oil to the skillet and heat the oil to medium. Add shallots/onions, minced garlic and a pinch of salt and freshly ground pepper, and cook until soft and golden brown. Remove from heat and set aside. (See notes for tips on how to keep the shallots/onions shape).
Place a rectangle piece of parchment paper (slightly bigger than your baking tray) or silicone baking mat on the counter. Top with the thawed puff pastry and roll out the puff pastry to form a rectangle. Turn up the edges to help form a crust (optional). Transfer puff pastry crust to a baking tray.
Cover the surface of the puff pastry crust with dijon mustard, grated vegan cheese, caramelized onions and half of the fresh thyme leaves.
Place onion tart into the oven and bake for around 20 minutes or until the pastry puff and turns golden brown.
Remove the onion tart form the oven and eight pieces, sprinkle with remaining fresh thyme leaves and serve while hot! Enjoy!
Store leftover in the fridge in an air-tight container. Reheat in the microwave (30 seconds) or in the oven (350F/176C oven) wrapped in foil to make sure it doesn’t burn or dry out.
We used shallots for the version you see in the photos. They are more subtle in flavor than other onions but tend to break down into a jammier texture when caramelized. They are also smaller, meaning you can leave them whole or cut into thick slices for an interesting texture.
To keep the shallots or onions in one piece (like in the pictures), place the thickly sliced shallots/onions into the skillet and try not to move them too much. Once golden brown on the bottom, flip and continue cooking. Alternatively you can break the shallots/onions up. The choice is yours!
Keywords: rustic onion tart, onion tart, French onion tart, The Plant Riot