Method

1. Mix together flour, margarine, and salt in a bowl with your fingertips or pulse in a food processor, until mixture resembles crumbs. Add 4 to 5 tbs ice water until mix forms a dough ball. Chill, wrapped in plastic wrap until firm, at least 1 hour. Make filling while dough chills.
2. Trim and cut off the long fennel stalks and fronds, then quarter bulbs lengthwise. Reserve the fennel fronds. Cut out and discard core, then cut each quarter crosswise into 1/2 cm thick slices. Reserve some of the fronds for garnishing. Prepare leeks, and wash well under cold water.
3. Mix together Marmite™, 1/4 cup water, oil, margarine, and pepper in a shallow frying pan and bring to a boil. Add fennel and leek slices and cook, covered, over medium to low heat, stirring occasionally, until fennel is tender, about 10 minutes. Remove and allow to cool.
4. Grease a fluted tart tin with a little spray oil. Roll out the chilled pastry on a floured surface until 3mm thick and big enough to fit the tart tin. Trim the edges and lightly prick bottom of shell all over with a fork, chill for 10 minutes.
5. Preheat oven to 180°C. Line shell with foil and fill with pie weights or baking beans, then put tart pan on a large baking sheet and bake until side of tart shell is set and edge is pale golden, about 15 to 20 minutes. Carefully remove pie weights and foil and bake shell until bottom is golden, 10 to 15 minutes more.
6. Whisk together eggs and yolk in a large bowl until foamy, then whisk in milk, sour cream, and season to taste. Transfer fennel mixture to shell with slotted spoon, spreading evenly. Pour egg mix over the fennel into shell and bake tart until set, 20 to 25 minutes. Cool on rack 15 minutes before serving. Garnish with fennel fronds to serve.