First cut the onion into cubes and sweat it in a good portion of olive oil in a large pan until it is nice and translucent.
Meanwhile you can boil 1 liter of water and dissolve the 4 tablespoons of vegetable stock in it. Then add the tube of tomato paste, stir well and set aside.
When the onion is ready, add the minced meat and fry, deglazing with a little red wine if necessary. Then add the vegetable stock to the pan and let it simmer on low heat. Here you can gradually add various additives such as Parmesan, garlic, fresh basil leaves (rub briefly in your hand, so the leaves reveal their aroma even better) and other spices - just as you like. I also add 1 cup of crème fraîche at this point. The longer the sauce simmer, the better it gets. Therefore, the sauce should simmer for at least 1 hour.
In the meantime, the feta cheese can be chopped into large cubes and the spinach can be thawed a little. Furthermore, the oven should be preheated to 200 degrees circulating air and the casserole dish should be greased with butter or margarine.
If the sauce meets the expectations (don`t be put off by a very spicy taste, this is put into perspective again in the finished lasagne), the baking dish can be filled. To do this, first pour in a layer of the sauce, then spread the feta and spinach in it. Then put the first layer of lasagne sheets, etc., until the sauce is completely processed. Then spread the cheese over the entire surface and put the mold in the oven for 40 minutes. When the lasagna has a golden brown layer, feasting is the order of the day.