Asparagus Ham and Mascarpone Tart

2 puff pastry sheets
30g unsalted butter
2 tsp olive oil, plus extra to drizzle
1 onion, finely chopped
150g sliced ham
2 tsp Dijon mustard
250g mascarpone cheese
2 bunches asparagus, trimmed, halved
1 egg, lightly beaten
¼ cup shaved parmesan

Preheat the oven to 180°C and line 2 trays with baking paper. Cut each pastry sheet into 4 squares, then lightly score a 2.5cm border around the edges of each square with a knife, without cutting all the way through. Place the squares on the trays and chill while you prepare the filling.
Melt butter with oil in a frypan over medium heat and cook onion for 2-3 minutes, stirring, until softened but not coloured. Cool slightly. Finely chop ham in a food processor. Add onion, mustard and mascarpone, then pulse to just combine. Season to taste with salt and pepper.
Spread mixture within the border of each pastry square, then top with asparagus pieces, alternating tips and ends. Drizzle with oil and season with salt and pepper. Brush pastry borders with egg, then bake tarts for 20-25 minutes until pastry is golden and asparagus is tender. Top with parmesan and serve with a rocket salad if desired.