Beat egg and sour cream in a small bowl.

Combine flour and salt in a stand mixer.

Add butter gradually, mixing until clumps form.

Add egg mixture and mix until just incorporated; do not overmix.

Knead dough briefly into a ball.

Wrap and refrigerate for at least 1 hour.

Divide dough in half.

Roll out one half on a floured surface to 1/8 inch thickness.

Cut out 12 rounds using a 3-inch cookie cutter.

Refrigerate rounds until ready to use.

Repeat with the other half of the dough.

Boil potatoes with salt until tender.

Drain and cool potatoes.

Grate potatoes.

Sauté onion in olive oil until translucent and browning.

Add zucchini and salt, cook for 2 minutes.

Add spinach and salt, cook for 2 minutes.

Drain mixture in a colander, pressing out excess liquid for 30 minutes.

Combine drained spinach mixture, grated potato, feta cheese, and salt in a bowl.

Mix gently.

Bring a large pot of water to boil.

Place a tablespoon of filling in the center of each dough round.

Fold dough into a half-moon shape, sealing edges firmly.

Boil dumplings for 3 minutes, then drain.

Set aside on wax paper.

Melt butter in a large sauté pan.

Cook dumplings in a single layer until golden brown and crispy on each side.

Spread sour cream on each plate.

Place lemon marmalade and dumplings on top.

Garnish with dill.