Heat oven to 190 degrees Celsius Roll out pastry to fit a deep 23cm loose bottomed tart tin. Wrap pastry carefully around rolling pin and unravel over tart tin. press gently around sides of tin. Chill 30 minutes, then line with greaseproof papter and fill with baking beans. Bake blind 15 minutes. Remove paper and beans and return tin to oven 5 minutes until pastry is golden. Remove tin from oven and set aside. Reduce oven temperature to 180 degrees Celsius.
Heat butter and oil in a large frying pan and add onions and potato. Cook gently 10-15 minutes, turning every so often until veg is golden and tender. Season to taste, then add garlic and thyme. Cook another 3 minutes. Tip filling into pastry case. Add goat's cheese and spread in an even layer.
Whisk eggs and creme fraiche together. Season to taste and pour into pastry case. Bake 30-35 minutes until lightly golden brown. You may have to cover edges of pastry with foil half-way through to prevent pastry burning. Tart is done when center is just set. Serve warm.