Note: all quantities and timings are for 4 servings; scale up by floor(number of people you are cooking for/4).

Preheat oven to 400F
In a large bowl, toss together eggplant, diced onion, unpeeled garlic, and 1 tbsp + 1 tsp olive oil; season with salt and pepper. Arrange in a single layer on a rimmed baking sheet, leaving empty space at one end. In same bowl, toss drained chickpeas with remaining 2 tsp oil. Transfer to empty space on sheet.
Roast until eggplant is golden and cooked through and chickpeas are slightly crunchy, about 35 minutes. When cool enough to handle, peel garlic.
In a medium pot, combine garlic, eggplant, onion, and stock. Bring mixture to a simmer over medium-high heat. With a potato masher or the back of a wooden spoon, mash some eggplant until soup is thick and chunky. Stir in chickpeas and season with salt and pepper.
Serve with oregano and yogurt as toppings.