Prepare a boiling water bath canner and four half pint jars.
Peel the eggplant and dice it into small cubes.
Peel the tomatoes and dice them into bits that are similarly sized to the eggplant.
Heat the olive oil in a low, wide, non-reactive pan and add the eggplant and tomatoes. Cook for 2-3 minutes and then add the red wine vinegar, garlic, sugar, salt, and black pepper.
Bring to a boil and then reduce the heat to a simmer. Cook, stirring often, for 20-25 minutes, until the liquid reduces and the the vegetables soften. If the eggplant isn't breaking down, use a potato masher to help the cubes meld into the spread.
Taste and adjust the salt and pepper levels, as necessary.
Remove the pot from the heat. Divide the citric acid between the four half pint jars and ladle the spread in on top. Use a chopstick or plastic bubbling tool to remove any trapped air bubbles and to work the citric acid into the spread.
Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 20 minutes.
When time is up, remove the jars from the canner and set them on a folded kitchen towel to cool.
When the jars are cool enough to handle, check the seals. Sealed jars are shelf stable for up a year. Any unsealed jars should be refrigerated and used promptly.