Peel the onions, cut into half rings and sauté in oil in a saucepan until they are light brown. Season with salt and pepper.
At the same time fry the minced meat in oil until crumbly. Add the tomatoes and let simmer until the liquid is reduced. Season to taste with salt, pepper and paprika.
Cut the washed tomatoes into slices that are not too thick.
Halve the burgers and put 6 lids back. Cut the remaining halves of the burger into cubes and place in a baking dish.
Place the warm minced meat mixture on top, as well as the onions that are still warm. Cover with the drained burger cucumbers and spread the tomato slices over them. Now cover the casserole with the 6 toast slices of cheese. The burger lids are placed on the cheese slices.
Optionally, you can coat the burger lid with a sauce from below before placing it on top. For this, depending on your taste, some of the burger sauce is mixed with ketchup and mustard.
Finally, sprinkle some grated cheese on the lid, if you like, you can also put cheese in the spaces in between.
Now the casserole is baked in the preheated oven on the middle rack. 200 ° C / approx. 15 min. / Top and bottom heat. The burger lids shouldn't get too dark.