Preheat oven to 180 ° C (160 ° C fan-forced) and lightly grease a 24cm round loose-bottomed fluted tart tin.
Place pastry on a lightly floured kitchen work surface. To check if the pastry is the correct size for the tart tin, place the tart tin over the pastry, there should be at least a 5cm border the whole way around.
Carefully line the tart tin with pastry, so that it sits 0.5cm above the top of the tin. Line the pastry with baking paper and fill to top of tin with baking weights or a dried pulse (eg. chickpeas) to help support the sides and weigh the base down during cooking. Place tart on a baking tray in preheated oven and bake for 20 minutes. Remove tart tin from oven, check pastry is lightly golden around the edges before removing beans and baking paper. Return tart tin to oven and cook for a further 10 minutes or until golden brown on base. Reduce oven temperature to 160°C.
Meanwhile prepare the fennel by removing the fennel fronds, reserving for later. Halve the fennel bulb and slice finely using a mandolin or very sharp knife.
Heat olive oil in a large frying pan over medium heat, add fennel, cover and cook for 10 minutes or until fennel is soft. Increase heat, cooking fennel until golden, season to taste with salt and pepper. Remove from heat, allow to cool.
In a small mixing bowl combine crème fraiche, eggs, horseradish, salt and pepper, whisking to combine.
Finely chop the reserved fennel fronds and stir into the egg mixture.
In a medium bowl combine cooked fennel, trout, parmesan and watercress, stirring gently to combine.
To assemble the tart, spoon the trout mixture into the prepared pastry case, then pour over the egg mixture.
Place tart in preheated oven, cook for 55 minutes or until the tart centre has just set. Remove tart from oven, allow to cool for 10 minutes before serving. This tart is delicious served either hot or cold.