Make the ratatouille: Heat the oil in a pan, cook the onion until tender but not brown, add tomato pulp, tomato paste, salt, pepper, sugar, cayenne pepper, herbs and stock granules, mix well.
Add eggplant, mix until coated with sauce and cook 5 minutes.
Add zucchini and pepper and cook for a further 10 minutes.
When the vegetables are cooked adjust salt and pepper to taste, the vegetables should be quite dry, if not add 1/2-1 tbsp flour and stir well while cooking another minute, so the liquid is absorbed.
Let cool 5 minutes, then add the mozzarella cubes to the ratatouille.
Roll out puff pastry so to have a 40x25 cm (I think it's about 20x12. 5 inches) rectangle, put the rectangle on a cookie sheet lined with baking paper, pour ratatouille and mozzarella mixture in the center of the rectangle and roll up, sealing the edges very well.
Brush with the beaten egg yolk, make a few cuts on the surface (so the steam can get out) and put in preheated oven 200°C for 25-30 minutes or until pastry is golden and crisp (you might want to put another lined cookie sheet on the lower rack as, depending on the freshness of the mozzarella you use, the strudel may produce some liquid and drip).
Let the strudel cool a little before slicing and serving.