Dice the onion and sauté with the minced meat, season with salt and pepper. Add the frozen spinach. Dice the feta cheese and add it to the pan.
When all the ingredients are hot and melted, add the sour cream.
Now alternate the lasagna sheets with the spinach mixture in a baking dish. The last layer should be spinach. Slice the tomatoes and place them on top of the lasagna. Bake in a preheated oven at 200 °C for about 40 minutes.