Preheat Oven: Preheat your oven to 400°F (200°C). Line a baking tray with parchment paper.
Prepare Puff Pastry: Roll out the puff pastry onto the prepared baking tray. Using a sharp knife, score a 1-inch border around the edges of the pastry without cutting all the way through.
Savory Base: In a small bowl, mix together the Dijon mustard, olive oil, minced garlic, and grated Parmesan cheese. Spread this mixture evenly over the center of the puff pastry, avoiding the scored border.
Add Asparagus: Arrange the asparagus spears in a single layer over the savory base. Season with salt and pepper.
Brush and Sprinkle: Brush the edges of the puff pastry with the beaten egg. Sprinkle the sesame seeds and poppy seeds around the border.
Bake: Bake in the preheated oven for 20-25 minutes, or until the pastry is golden and puffed, and the asparagus is tender and slightly charred.
Serve: Remove the tart from the oven and let it cool slightly. Top with mixed salad leaves and tear the burrata or fresh mozzarella over the tart.