In a large saucepan, heat oil over medium until shimmering. Add onion and cook, stirring occasionally, until softened and translucent, 5 to 6 minutes. Add garlic and cook, stirring constantly, until fragrant, about 30 seconds.
Stir in chickpeas, salt, and vegetable stock. Bring to a boil over medium-high heat, then reduce heat to medium-low and simmer, stirring occasionally, until flavors meld and chickpeas are very tender, about 15 minutes.
Add tahini to a medium heatproof bowl. Ladle about 2 cups of the hot soup into bowl with tahini and whisk until smooth and well combined; set aside.
Remove saucepan from heat. Using an immersion blender, process soup in saucepot until mostly smooth. (Alternatively, carefully transfer the soup to a traditional blender and process until smooth, then return to the pot and proceed with recipe; take care when blending hot liquids.) Stir in reserved tahini-soup mixture, lemon juice, and dill. For a thinner soup, add additional stock as needed to reach desired consistency.
Ladle soup evenly into 4 bowls. Garnish with dill, sumac, lemon zest, and a drizzle of olive oil. Serve immediately.
