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1
Place onions, celery, and tomatoes in a large pot; pour in enough water to just cover vegetables; bring to a boil. Reduce heat and simmer until tomatoes are tender, 20 to 30 minutes. Pour tomato mixture through a strainer or food mill, discard solids, and return liquid to the pot; stir sugar and salt into liquid.
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2
Blend butter and flour together in a blender until smooth and creamy; add tomato juice. Blend until smooth. Pour butter mixture into liquid in the pot. Cook and stir over medium heat just until tomato soup is hot.
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3
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the tomato soup into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
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4
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars into the boiling water using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. Bring the water to a rolling boil, cover the pot, and process for 20 to 30 minutes.
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5
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area, and wait at least 12 hours before opening.
