Preheat the oven to 220C/fan200C/Gas7. Use the pastry to line the base and sides of a swiss roll tin 23x33cm. Heat the oil in a large frying pan; fry the onions for 8-10 minutes until just beginning to caramelise. Stir in the garlic and most of the thyme; cook for a further 2 minutes. Scatter half into the case.
Parboil the potatoes in salted boiling water for 4-5 minutes; drain well and arrange in the case. Scatter over the remaining onions.
Beat together the eggs, creme fraiche and mustard. Season well and pour over the top. Scatter over the rest of the thyme and bake the tart for 20 minutes, until the filling has set and is golden.