Place a medium saucepan over medium heat. Add 2 tablespoons oil. Once hot, add onions and chickpeas and cook just until chickpeas are heated through, stirring frequently, about 5 minutes. Add artichoke hearts and olives and stir 1 minute more. Remove from heat.

Halve lemons crosswise. Set one half aside. Juice remaining 1½ lemons into chickpea mixture, straining out seeds. Cut reserved lemon half lengthwise into quarters; then cut each quarter crosswise into paper-thin slices. Toss lemon slices and remaining 4 tablespoons olive oil with chickpea mixture. Season to taste with sea salt and freshly ground black pepper.