Lightly brush a 19cm slab pan with a little of the olive oil to grease.
Preheat oven to 180°C. Melt butter in a frying pan over medium-high heat. Add onions and cook, stirring, for 5 minutes or until soft. Add vinegar and sugar, and stir to combine. Cook, stirring occasionally, for 8-10 minutes or until liquid evaporates and onions caramelise. Set aside for 5 minutes to cool slightly.
Whisk together eggs, cream and salt in a jug until combined.
Place filo on a clean work surface. Cover with a dry tea towel and then a damp tea towel to prevent drying. Brush 1 sheet of filo with a little olive oil and fold in half crossways. Place filo lengthways in pan to line base and 2 opposite sides. Lightly brush another sheet of filo and fold in half crossways. Place filo crossways in pan to line base and 2 opposite sides. Repeat, alternating, with remaining filo and olive oil. Spread onions over top and pour over cream mixture. Sprinkle with pine nuts.
Bake tart, covered with foil, in preheated oven, for 15 minutes. Uncover and bake for 15 minutes or until tart is set. Set aside in pan for 10 minutes to cool. Cut into 4 equal pieces to serve.