Sprinkle eggplant halves with salt and set aside for about 30 minutes, then rinse and wipe dry with kitchen paper (this helps remove bitterness). If using whole eggplant make a slit lengthways with sharp knife and gently pull open to expose eggplant pulp.
Heat half the olive oil in a frying pan over medium heat then fry the eggplants on both sides. The cut side should be golden. Remove from pan, drain on kitchen paper and set aside to cool.
Remove about half the pulp from the cooled eggplant halves with a spoon, then chop and set aside. If using whole eggplant gently remove some pulp, taking care not to tear open the eggplant too much.
Add remaining olive oil to frying pan and sautè onion until translucent.
Add garlic, eggplant pulp and tomatoes, mix well and bring to a simmer.
Add blackcurrants and walnuts, season to taste and stir thoroughly.
Arrange reserved eggplant shells in an ovenproof dish, spoon in cooked vegetable mixture. Sprinkle with feta.