Preheat the oven to 400°F.

Roll out the puff pastry sheets on a lightly floured surface to fit a 9-inch tart pan. Place the pastry in the pan and trim any excess.

In a skillet over medium heat, cook the chopped bacon until crispy. Remove the bacon and drain on paper towels, leaving about 1 tablespoon of bacon fat in the skillet.

Add the diced onion and sliced mushrooms to the skillet and sauté until the onions are translucent and the mushrooms are tender, about 5-7 minutes. Season with salt and pepper.

In a mixing bowl, whisk together the eggs, heavy cream, Dijon mustard, and a pinch of salt and pepper until well combined.

Stir in the cooked bacon, sautéed mushrooms, and onions. Fold in the shredded Gruyère cheese.

Pour the egg mixture into the prepared puff pastry shell.

Bake in the preheated oven for 25-30 minutes, or until the filling is set and the top is golden brown.

Let it cool for a few minutes before slicing.

Garnish with fresh parsley before serving.