- 1
Preheat oven to 400°F. Line two baking sheets with parchment paper and set aside.
- 2
Heat 1 Tbsp. olive oil in a large skillet over medium heat. Add onions and salt; sauté onions until they begin to brown. Reduce heat to low and cook, stirring frequently, until golden and caramelized, about 25 minutes. Transfer onions to a large bowl and set aside.
- 3
Heat remaining 1 Tbsp. olive oil in skillet over medium-low heat. Add mushrooms and sauté, stirring occasionally, until tender, about 5 minutes. Add garlic, thyme and black pepper; sauté for 1 more minute or until garlic is fragrant. Transfer to bowl with onions, add cheese and stir to combine.
- 4
On a lightly floured surface, roll out puff pastry sheets. With a 2-inch round cookie cutter, cut each pastry sheet into 16 rounds.
- 5
Transfer the rounds to prepared baking sheets. Brush lightly with beaten egg. Lightly prick each pastry round with a fork to allow air to escape. Top each round with ¼ tsp. mustard and 1 Tbsp. of mushroom filling.
- 6
Bake for 25 minutes, or until pastry is golden and crisp. Rotate pans halfway through baking time. Let bites cool for 5 minutes before garnishing with parsley; serve warm or at room temperature.
