Step 1

Start by pre-heating your oven to 200C (400F) and line two baking sheets with parchment paper.

Step 2

Heat vegetable oil in a skillet over medium heat, and cook the onions with a little salt for 20 minutes. Stir occasionally. Add in the mushrooms and cook 5 more minutes, until onion is golden brown and mushroom is tender. Place the mixture in a bowl.

Step 3

Melt butter in the same skillet and add apple pieces and sugar. Cook until softened, about 10 minutes. Add into the bowl with onion and mushrooms. Add parmesan, chives and rosemary into the bowl and mix. Season with salt and pepper, to taste.

Step 4

Using a pizza cutter or sharp knife, cut the puff pastry into 6 cm squares (2.3 in), and 2.5 mm thick (0.1 in). It doesn't have to be scientific. Place them on the prepared baking sheets and brush with a beaten egg.

Step 5

Place a heaping teaspoon of filling in the middle of each puff pastry bite. Sprinkle with more parmesan, if you want. Bake for 20 minutes. Let the tarts cool a little before serving. Drizzle with more chives, if desired.