Preheat the oven to 180°C/ 160°C (fan forced) . Line 22cm loose bottom flan tin (round or square) with the pastry, and trim the edges. Blind bake for 15 minutes. Remove paper and weights and bake a further 10 minutes. Cool slightly

Meanwhile, heat the oil in a large frying pan, and add the onions. Cook over medium heat for 15 minutes, stirring occasionally, until deep golden brown. Steam or microwave sweet potato until tender. Set aside to cool

Fill the tart shell with the onions, sweet potato and feta. Pour eggs over and sprinkle with thyme. Season with salt and pepper. Bake for 40-45 minutes, or until the egg has set. Serve warm or at room temperature