Wash the broccoli and divide into florets. Peel the onions and cut into thin rings.
Wash the asparagus, peel the bottom half and cut off the woody ends. Cut the asparagus into 5cm pieces.
Bring the stock to the boil in a saucepan. Add the asparagus and cook, covered, over a medium heat for 5 minutes. Remove and allow to cool.
Add the onions to the stock, bring to the boil and cook for 2 minutes. Add the broccoli, boil again and cook for 1 minute.
Pour the Stock through a sieve into a bowl, remove the vegetables and allow to cool.
Meanwhile, mix the rye flour, flour, baking powder and 1/2 tsp salt in a bowl. Add 5 tbsp cold water and the chilled butter in small pieces, and mix to a smooth dough.
Roll out about two-thirds of the dough on a floured surface to a 30cm round. Wrap the remaining dough in cling film, and place in the fridge.
Lightly grease a 28cm tart tin. Line with the rolled out pastry and prick the base several times with a fork. Bake in the preheated oven at 200°C (180°C fan, gas 6) for 10-15 minutes. Remove from the oven and allow to cool slightly.
Meanwhile, mix the eggs with the sour cream and 100ml of the cooled stock. Season with salt and pepper.
Take the remaining pastry from the fridge and shape into 2 thin rolls, about 42cm long. Press the rolls, one on top of the other, around the edge of the tart.
Add the asparagus, broccoli and onions to the tart tin and pour in the egg mixture. Bake the tart in a preheated oven at 200°C (180°C fan, gas 6) for 30 minutes. Season with pepper and serve.