Peel and finely dice the onions and garlic. Heat 1/2 tbsp. oil in a pot. Fry half of the onions in it. Add frozen spinach and 100 ml water, cover and allow to defrost over medium heat for 14-16 minutes.
Then openly simmer for 5-6 minutes until the liquid has evaporated. Season with salt and pepper.
Heat 1 tablespoon of oil in a frying pan. Fry the minced meat in it until crumbly. Add garlic and rest of onions, fry. Stir in tomato paste, sweat briefly. Stir in strained tomatoes and herbs, bring to the boil and simmer covered for about 5 minutes.
Season to taste with salt, pepper and 1 tsp. sugar.
For the béchamel sauce melt butter. Dust with flour and briefly sauté lightly. Add 1/4 l water and milk while stirring. Stir in the stock, bring to the boil and simmer for about 5 minutes while stirring. Season to taste with salt and pepper.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Cover the bottom of a square casserole dish (approx. 20x30 cm) with a little béchamel sauce. Place 3-3 1/2 lasagne plates on top. Alternately layer spinach, sauce and lasagne, then mince, sauce and lasagne.
Continue in order until everything is used up. Finish with a layer of lasagna and sauce. Grate the cheese directly over the lasagne and bake in a hot oven for 40-45 minutes.