Step 1: Prepare the Puff Pastry

• Preheat your oven to 200°C (400°F). Line a baking sheet with parchment paper.
• Roll out the puff pastry on a lightly floured surface to about 1/4-inch thickness.
• Using a round cutter (or a glass), cut out circles of pastry and place them on the prepared baking sheet.
• Score a smaller circle inside each round without cutting through completely to create a border. Prick the center with a fork to prevent puffing.

Step 2: Sauté the Onions and Mushrooms

• Heat olive oil in a large skillet over medium heat.
• Add the chopped onion and cook for 2-3 minutes until translucent.
• Add the minced garlic and cook for another minute until fragrant.
• Add the sliced mushrooms and cook for about 5-7 minutes until they release their moisture and start to brown.

Step 3: Add Spinach and Rosemary

• Stir in the fresh rosemary and cook for another minute.
• Add the baby spinach and cook until wilted, about 2 minutes.
• Season with salt and black pepper to taste. Remove the mixture from heat and let it cool slightly.

Step 4: Assemble the Tartlets

• Place about a tablespoon of the mushroom-spinach mixture in the center of each pastry circle.
• Top with crumbled feta cheese.
• Brush the edges of the pastry with the beaten egg for a golden finish.

Step 5: Bake the Tartlets

• Bake in the preheated oven for 15-18 minutes or until the pastry is puffed and golden brown.
• While the tartlets are baking, prepare the balsamic glaze.

Step 6: Make the Balsamic Glaze

• In a small saucepan, heat the balsamic vinegar over medium heat until it starts to simmer.
• Reduce the heat to low and let it simmer for 10-15 minutes until it reduces by half and thickens.
• Stir in honey for a touch of sweetness, if desired.

Step 7: Drizzle and Serve

• Remove the tartlets from the oven and let them cool slightly.
• Drizzle with the balsamic glaze before serving.