Preheat oven to 210C. Line six 8cm tart cases with pastry and prick the base with a fork. Freeze for 30 minutes. Bake frozen tart shells for 10-12 mins or until dry and light golden, then cool on a wire rack.
Heat 2 tbsp of olive oil in a pan and add onions and garlic and cook over medium heat for about 5 mins, stirring occasionally, until softened.
Sprinkle in sugar and cook for 4-5 mins until caramelised. Add vinegar and season with salt and pepper. Remove from heat.
Toss mushrooms in a bowl with remaining oil. Transfer to an ovenproof dish, dot with butter, season and roast for 20-25 mins or until mushrooms are tender. Drain on kitchen paper.
Place onions in the bottom of warmed shells, add warm mushrooms and top with shaved parmesan.
Serve tarts on a bed of rocket and drizzle with extra balsamic vinegar.
- Cuisine - Italian
- Course - Lunch