Heat the oven to 425ºF. Cut the eggplant in half lengthwise. Oil the cut side lightly with olive oil and sprinkle generously with kosher salt. Roast, cut side up, for 1 hour, or until very soft and blackened around the edges. The eggplant halves will be puffy and expanded, but will collapse as they cool. Let cool for 1 hour.
Scoop the flesh away from the skin and discard the skin. Put the eggplant flesh in in a fine mesh strainer. Press out and drain any juice that will come out. Transfer to a mortar or food processor. Add the garlic, lemon juice, tahini, and black pepper and blend thoroughly until very smooth. Taste and add additional lemon juice, olive oil, salt, or pepper to taste, if desired.
Refrigerate for at least one hour to let the flavors blend. Before serving dust lightly with ground sumac. Serve with warm pita bread, baked chips, or chopped raw vegetables.
Refrigerate any leftovers in a covered container for up to five days.