Clean, wash and cut the leek into rings. Heat 3 tablespoons of oil and 20 g butter in a saucepan. Steam the leek in it for about 10 minutes until transparent. Clean, wash and roughly chop the spinach, except for a few leaves for garnishing.
Wash the dill, dab dry and cut finely. Crumble the feta cheese. Stir the spinach, dill, 5 tablespoons of oil and 1 whisked egg into the hot leek. Let it cool down. Melt 100 g butter in a saucepan.
Whisk the rest of the egg. Spread out the dough sheets and cut them into 24 rectangles of approx. 20 x 30 cm. Spread 12 dough rectangles first with butter, then with egg. Cover each with another dough sheet.
Spread some spinach filling on each of the dough sheets, leaving a 2-3 cm wide rim all around. Roll up into thick cigars, folding the sides slightly inwards. Spread oil on a large flat roasting pan or fat pan.
Place spinach rolls side by side in the roaster. Bake in the preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level: 3) for 25-30 minutes. Garnish with lemon slices and remaining spinach.