Heat half the olive oil in a frying pan over medium-high heat, add onion and cook for 3 minutes or until soft. Deglaze with vinegar, add sugar and cook for another 2 minutes or until liquid is absorbed. Remove from heat, add olives, oregano and season to taste with sea salt and freshly ground black pepper.
Preheat oven to 200C. Lightly brush a 20cm-diameter loose-bottomed round flan ring with butter. Working with one sheet of pastry at a time, brush with butter, fold in half, brush top with butter and use to line flan ring. Repeat with remaining sheets, placing each offset to the last, and fold overhanging pastry into centre. Spread onion mixture over base, scatter with tomato and feta and season to taste. Bake for 10 minutes or until pastry is crisp and serve immediately or at room temperature with a green salad.