2.Place sweet potato on an oven tray; drizzle with oil. Roast for 30 minutes or until tender.
3.Meanwhile, to make pastry, process flour, thyme and butter until crumbly. Add egg yolk and 2 tablespoons chilled water; process until ingredients come together. Wrap in plastic wrap; refrigerate for 30 minutes.
4.Roll pastry between sheets of baking paper until large enough to line a 24cm (9½in) round loose-based tart tin. Lift pastry into tin, press into side, then trim edge; prick base all over with a fork. Cover; refrigerate for 20 minutes.
5.Reduce oven to 190°C/375°F.
6.Place tin on an oven tray; line with baking paper, fill with dried beans or rice. Bake for 15 minutes. Remove paper and beans; bake for a further 10 minutes or until browned lightly. Cool on tray.
7.Sweet potato can be roasted and pastry case made 2 days ahead. Store sweet potato in an airtight container in the fridge. Store pastry case in the tin, covered, at room temperature.
Tart can be baked a day ahead. Store covered, in the fridge.
8.Preheat oven to 190°C/375°F. Whisk eggs, cream and cheese in a medium jug until combined. Stir in broad beans; season. Place sweet potato in pastry case; pour over egg mixture and scatter with fetta.
9.Bake tart for 35 minutes or until filling is just set. If you are reheating cooked tart, bake for 10 minutes or until hot.
10.Serve tart topped with baby salad leaves, extra grated cheese and extra broad beans.
You can also make six individual tarts in 3cm deep, 10cm round loose-based tart tins; bake for 25 minutes.
swap When in season you can swap the sweet potato for kohlrabi or swede.