Preheat the oven to 180°C.
Place the grated sweet potatoes in a clean cloth or use your hands and squeeze all of the liquid out of it, this may have to be done in batches.
Brush a 20 x 10cm baking tray with melted butter - ensure all of the sides are covered.
In a bowl, whisk 1 egg with salt and pepper then mix in the grated sweet potato.
Press the sweet potato mixture into the base and sides of the tray and bake for 20 minutes. Allow to cool.
While the base bakes, heat up some butter in a pan and fry off the spinach with salt and pepper.
In another bowl whisk the remaining 9 eggs together with the milk, season with salt and pepper and mix in the spinach leaves.
Pour the egg mixture into the base, place the baby leeks in the tray and crumble the feta over.
Place in the oven and bake for around 15 minutes or until the eggs are cooked through but the tart wobbles slightly. Remove from the oven and allow to cool slightly before slicing.
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