Caramelized Onion and Blue Cheese Tart

Instructions

1. Caramelize the onions
   Heat butter and olive oil in a large, heavy-bottomed pan over medium-low heat. Add the sliced onions with a pinch of salt and cook slowly, stirring occasionally, for about 45 minutes until deeply golden and sweet. Add the balsamic vinegar and brown sugar, and cook for another 10 minutes until sticky and caramelized. Stir in the fresh thyme leaves and set aside to cool.
2. Prepare the pastry
   Preheat oven to 375°F (190°C). Roll out the puff pastry on a lightly floured surface and use it to line a 9-inch (23cm) tart pan with a removable bottom. Trim any excess pastry and prick the base all over with a fork. Line with parchment paper, fill with baking beans, and blind bake for 15 minutes. Remove the beans and paper and bake for another 5 minutes until the base is lightly golden.
3. Make the filling
   In a bowl, whisk together the eggs, heavy cream, and Dijon mustard. Season with salt and pepper. Spread the cooled caramelized onions evenly over the base of the partially baked tart shell. Scatter two-thirds of the crumbled blue cheese over the onions.
4. Bake the tart
   Pour the egg and cream mixture over the onions and cheese. Sprinkle the remaining blue cheese and the chopped walnuts (if using) on top. Bake in the preheated oven for 25-30 minutes until the filling is set and the top is golden brown.
5. Rest and serve
   Allow the tart to rest for 10-15 minutes before removing from the pan. Garnish with additional fresh thyme leaves. Serve warm or at room temperature with a simple green salad on the side.