Pre heat oven to 180°C (350°F)
Heat butter in an oven proof fry pan. (25cm) Add the fennels and stir for a few minutes. Then add the sugar, salt and pepper, red wine vinegard and stock. Cover and simmer for about 10 minutes, being careful not to burn.
Add the piece of pastry on top and tuck around the edges.
Bake in oven at 180°C for 30 to 35 mins till the top of pastry is all crisp.
Flip out the tart onto a plate and drizzle with balsamic glaze and decorate with a few fresh herbs if desired.