Prepare the marinade. Pour 2 cups of filtered water into a medium saucepan. Place on the fire and bring to a boil. Add salt and sugar. Stir until the ingredients dissolve. Turn off the heat. Add the swollen gelatin. Stir thoroughly until dissolved. Add vinegar to the marinade. Stir.
Marinate the tomatoes. Place black peppercorns on the bottom of a sterile jar. Add bay leaf and dill. Add garlic. Place tomatoes and onions on top, alternating layers. Pour the marinade over the vegetables so that there is 1.5 cm left to the top.
Marinate the tomatoes. Line the bottom of a large deep saucepan with a towel. Place the jars in it. Pour in water so that it reaches the "shoulders" of the jars. Put on the fire and bring to a boil, cover the jars with lids, but do not screw them on. Reduce the heat to low. Boil for 15 minutes. Remove the jars and roll up the lids. Turn the jars upside down. Wrap them in a warm blanket. Leave for a day.