In a large mixing bowl, combine the all-purpose flour and salt.
Make a well in the center and add the egg, warm water, and vegetable oil.
Mix until a dough forms, then knead on a floured surface for about 10 minutes until the dough is smooth and elastic.
Wrap the dough in plastic wrap and let it rest for at least 30 minutes.
In a medium bowl, combine the cottage cheese or ricotta cheese, egg, sugar, vanilla extract, and lemon zest (if using).
If you prefer a firmer filling, add the bread crumbs. Mix well until all ingredients are combined.
On a lightly floured surface, roll out the rested dough into a large, thin rectangle (about 1/8 inch thick).
Spread the cheese filling evenly over the rolled-out dough, leaving a small border around the edges.
Starting from one of the long edges, carefully roll the dough into a tight log.
Pinch the seams to seal and tuck the ends under to secure.
Bring a large pot of salted water to a gentle boil.
Wrap the dough log in a clean kitchen towel and tie the ends securely with kitchen twine.
Carefully place the wrapped dough log in the boiling water and cook for about 25-30 minutes.
In a small skillet, melt the butter over medium heat.
Add the bread crumbs and cook, stirring frequently, until golden brown and crispy.
Carefully remove the cooked štruklji from the water and let it cool slightly.
Unwrap the kitchen towel and slice the štruklji into thick rounds.
Arrange the slices on a serving plate and drizzle with melted butter.
Sprinkle the toasted bread crumbs over the top.
Optionally, dust with powdered sugar and serve with a dollop of sour cream.
Enjoy your meal!