Preheat oven to 425°F and line a rimmed baking sheet with foil.
Wash and pat dry the eggplants. Pierce all over with a fork like you would a roasted potato. Drizzle with olive oil, and roll in oil to coat. Roast for an hour to an hour and half, or until eggplants are tender and collapsing.
Allow eggplant to cool to room temperature. Using a knife and spoon, cut the eggplants in half and scoop flesh out, discarding the skins. Place eggplant pulp in a fine mesh sieve over a medium-sized bowl and allow to drain for 15 minutes.
In the meantime, remove the seeds and stem from your serrano and set the pepper aside. Crush garlic with the flat side of a knife and discard the skin. Juice the lemon and set aside.
Place drained eggplant, garlic, serrano, lemon juice, and salt in a food processor or blender and blitz until smooth and creamy.
Plate with a drizzle of olive oil, a dash of sumac, and feta cheese crumbles. Serve immediately. Keeps up to 4 days in the fridge in an air tight container.