For the filling, wash the vegetables and heat a large pot of salted water. Once the water boils, cook the vegetables in it for about 2 minutes, then strain and drain.
Preheat the oven to 180 °C.
For the béchamel sauce, melt the butter in a saucepan, gradually stir in the flour and add the milk. Stir constantly so that no lumps form.
Now peel and finely chop the garlic and onions.
Then mix the vegetables with the béchamel sauce and the finely chopped garlic and onions. Salt, pepper and season with a little cumin.
Place the puff pastry on a kitchen towel and spread the vegetable filling in the lower third. Gently fold in the sides of the strudel and form it into a roll. Close the ends well so that nothing leaks out.
Cover a baking tray with baking paper and place the strudel on it. Brush with the beaten egg and bake in the oven for about 25-30 minutes.
For the herb sauce, mix the yogurt with the sour cream. Chop the chives, dill and parsley and stir into the yogurt sauce. Add salt, pepper and a pinch of nutmeg to taste.
The vegetable strudel with herb sauce can be served warm, but is also highly recommended cold as an office snack.