1. Preheat oven to 475°F and line 2 rimmed baking sheets with parchment paper (make sure the parchment fits inside the pan; at this high temperature, any overhang could burn). Peel the eggplants, leaving stems attached; on each, poke the center of the base a few times with a fork or cake tester and divide between the 2 pans.
2. Toss each eggplant with 2 tablespoons olive oil and a pinch of salt, massaging the salt into the flesh a little as you work. Create space between the 2 eggplants on each pan and roast for 40 to 50 minutes, turning the eggplants every 10 to 15 minutes to get even browning on all sides. Remove from oven and set aside.
3. Meanwhile, heat a 12- to 14-inch sauté pan over medium-high heat and add the remaining 1/4 cup olive oil. Once the oil is shimmering, add the chickpeas and a pinch of salt. Pan-fry the chickpeas, stirring occasionally, for 8 to 10 minutes, or until crispy (as they crisp up, afew may pop out of the pan like popcorn). Lower the heat slightly, add the sumac, and cook just 1 minute more. Taste and add more salt, as needed.
4. Season each eggplant with a squeeze of lemon. Place on warmed plates or in shallow bowls and top the center of the base of each eggplant with a 1/4 cup tahini sauce. Top the tahini sauce with a heaping 1/2 cup sumac-coated, crispy chickpeas. 
5. Drizzle a little olive oil around the sides of the eggplant and top the olive oil with a sprinkling of chopped parsley.
6. Serve at once.

For the Tahini Sauce: 

1. Mince the garlic, if using, with half the salt until a smooth paste forms. 
2. In a medium mixing bowl, whisk together the garlic paste with the lemon juice and the remaining salt. 
3. Let sit for 10 to 15 minutes; this mellows the garlic and dissolves the salt. Even if you omit the garlic, let the salt sit in the acid for a few minutes. 
4. If the oil has separated in your tahini, stir it back together, then spoon approximately 1 cup (I say “approximately” because there is no need to dirty a measuring cup with thick, oily tahini) into a medium bowl. 
5. Use a whisk to start beating in cold water, 1/4 cup or so at a time, until the mixture becomes smooth and creamy. At some point you’ll think you’re doing it wrong because it will look a mess—don’t fear. Soon you’ll see it start to emulsify, lighten in color, and become easier to stir. It will probably take about 3/4 cup water total. 
6. Thin the tahini sauce with more water until you reach your desired consistency. Then taste and adjust the seasoning with more salt or lemon juice, as needed. 
7. Use immediately or refrigerate in an airtight container for up to 4 days.