Fontina, caramelised onion and potato tart

Ingredients

• 1 tablespoon olive oil
• 3 large onions, finely sliced
• 1/4 cup (55g) brown sugar
• 2 tablespoon balsamic vinegar
• 500 gram potatoes, cut into 1.5cm pieces, parboiled
• 2 sheets frozen puff pastry, thawed
• 2 eggs
• 100 millilitre light thickened cream
• 100 gram grated fontina cheese
• 2 teaspoon thyme leaves, plus extra small sprigs
• salad greens, to serve