Assemble dish: Halve and very thinly slice second half of first lemon, fishing out any lemon seeds. In a 9×13-inch baking dish (or lasagna pan), combine lemon slices, 3 tablespoons olive oil, onion, garlic, remaining 1/2 teaspoon oregano, 1 teaspoon kosher salt, and many grinds of black pepper. Add potato wedges and chickpeas and stir until everything is evenly mixed and coated. Remove chicken thighs from bowl and place them atop the potato-chickpea mixture, skin side-up. Pour any juices left in the bowl over the potatoes and chickpeas. Pat the chicken skin dry and drizzle it lightly with more olive oil.