Wash, cut and salt the eggplants. Let them drain in the strainer for about half to one hour (it doesn't matter if you leave them for a longer time) and continue by rinsing and sqeezing them lightly. That doesn't only degorge them but also makes them not absorb a lot of oil during cooking.

Heat the oil and lightly sauté the onion, garlic and eggplants. 

Add the tomatoes or the tomato juice and if necessary (depending on how much water the tomatoes hold or how concentrated the juice is) add a bit of hot water. 

Let boil for about 20 minutes to half an hour, taking care that it doesn't stick. 

When the eggplants are almost done add a bit of boiling water and proceed by adding the trahana rubbing it by hand (it usually comes in big chunks that are very easy to break). During this, stand over the pot stirring and adding as much water it needs (the trahana boils in about five to ten minutes). 

Add salt and pepper according to taste and serve.