Preheat the oven to 180°C fan-forced (200°C conventional).
Sprinkle the pastry sheet with the thyme leaves. Roll it out to fit a pie dish 27 cm in diameter and 5 cm deep. Prick the base all over with a fork. Line with baking paper and fill with baking beans or rice, then blind-bake for 10–12 minutes or until lightly golden.
Remove from the oven and remove the paper and baking beans. Brush the base with the beaten egg, return to the oven and bake for 6 minutes until golden and crisp.
Reduce the oven temperature to 150°C.
Place the kale in a large heavy-based saucepan over medium heat. Place a lid on top and allow to soften and wilt for 15–20 minutes.
Heat the olive oil in a frying pan over medium heat and add the onion. Fry for 4–5 minutes until soft. Add the kale and mustard seeds and cook for a further minute. Season with salt and pepper to taste. Spread the kale mix over the tart case, then sprinkle over the parsley and blue cheese.
In a bowl, whisk the eggs, yolks, cream and sour cream. Season with salt and pepper to taste, then pour into the tart case. Place the prosciutto strips on top.
Bake for 40–50 minutes or until light golden and just set with a slight wobble. Leave to stand for 10 minutes, then serve.