To cook the eggplant in the pressure cooker, put the peeled, quartered eggplant in a steamer basket, place it in the cooker, and add just enough hot water to cover the bottom of the cooker but not so much that it touches the eggplant (about a cup or two). Seal the cooker, bring to high pressure, and cook for 2 minutes after it reaches high pressure. Use a quick-release method to reduce the pressure. (Follow the instructions here to cook the eggplant in the oven.)
Drop the garlic into a running food processor to chop it. Add the eggplant, and process until it is almost smooth. Add the remaining ingredients, and process until smooth. Check the seasonings, and add more as necessary. (If the eggplant seems bitter, a pinch or two of sugar helps.) Serve sprinkled with sumac.