Clean and wash the flounder. Sterilize jars and lids. Also peel and wash the onions and carrots well. Prepare the remaining ingredients.
Cut the flounder fillet into small portions, transfer to a container, add salt and leave for 40 minutes. Grate the carrots on a coarse grater, cut the onion into thin half rings.
Place onions and carrots in a well-heated frying pan and fry, add tomato paste and pour in 100 g of water. Then add salt and sugar to taste and simmer for another 10 minutes.
Place allspice and bay leaves at the bottom of the prepared jars. Place pieces of salted flounder and pour vegetable sauce over it. Place the jars in a cold oven and cook the canned food at 120 °C for 4 hours, then roll up.
Carefully place the flounder pieces on a plate and pour the sauce on top. Serve with olives and fresh cucumbers.