Wash the eggplants well and make a slit on the side.
Drizzle lightly with olive oil and bake at 200° C for 45 minutes to an hour until they are soft and fully cooked (depending on the size of the eggplant).
Finely chop the onion and garlic.
Fry the onion until soft, then add the garlic and cook for another minute.
In the meantime chop the bell pepper and cut the cherry tomatoes in half and add them to the pan.
Add the oregano, mix well and cook for another couple of minutes.
Remove from the heat and add the drained tuna, mustard and season with salt and pepper.
Mix well and set aside.
When the eggplant is ready, let it cool down for about 10 minutes to be easy to handle.
Open them with the help of a spoon and form a pocket. Stuff them with the filling.
Place the tomato sauce at the bottom of your baking dish and place the stuffed eggplant on top. You can use a ready made sauce like a basil one instead of plain tomato.
Add some crumbled feta cheese on top and bake again for about 10 minutes or until the cheese is lightly browned.