Whisk together flour, water, oil and salt and knead into a smooth, pliable dough. Shape into a ball and rest wrapped in plastic wrap at room temperature for at least thirty minutes.
Peel and chop the onion. Sauté in butter (1). Add long grain rice and fry briefly. Extinguish with the soup. Cook the long grain rice al dente, stirring frequently. Cool down a bit.
While the long grain rice is cooking, cut the corn kernels from the cobs.
Cut the savoy cabbage in half, remove the stem and coarse leaf veins, then cut the vegetables into strips. Cut the peppers in half, remove the seeds and cut them into small cubes. Cook all the vegetables in a little salted water or possibly over steam until al dente. Drain and cool a little, then mix with the long grain rice.
Cut the salmon into cubes of about two centimeters. Chop the dill. Mix both together with the sour half cream or yogurt into the vegetable rice. Season with salt and pepper.
Roll out the strudel dough into a rectangle on a floured surface with a rolling pin. Now place a kitchen towel underneath and pull out the dough with the backs of your hands section by section until paper-thin. Cut away thicker parts of the edges.
Spread the filling evenly over 2/3 of the dough surface, leaving a margin of the three cm free. Brush edges of dough with water and fold up over filling. Roll up the strudel by lifting the cloth underneath and slide onto a baking sheet lined with parchment paper