In a large pot, heat ⅓ cup of the olive oil over medium heat. Add the minced onion and garlic and cook until wilted and soft, about 7 to 8 minutes.
Add the drained chickpeas and enough water to cover them by about 2 inches. Bring to a simmer, partially cover the pot, and cook for 2½ to 3 hours, until the chickpeas are very tender.
Using an immersion blender, puree approximately 1 cup of the mixture directly in the pot (or transfer 1 cup to a food processor, puree, and return it to the pot).
Season with salt to taste, bring back to a gentle simmer, and stir in the remaining ⅓ cup olive oil, the dried oregano, and the lemon juice.
Serve warm.
Notes
Adjust the water level during cooking: if the soup gets too thick, add a little hot water to reach your preferred consistency.
The blending step gives the soup a creamy body without making it fully puréed — you want some whole chickpeas to remain.
If you like, finish each bowl with a drizzle of extra olive oil and a sprinkling of oregano.