Place the butter and the olive oil in a large skillet over medium heat. As the butter melts, stir it with the oil. As soon as the fats coat the bottom of the pan, add the leeks, stir to coat them with the butter and the oil, season lightly with salt, cover, and cook, stirring regularly, until the leeks are tender and slightly golden on the edges, about 8 minutes. Add the squash, the lemon zest, and 3 tablespoons of water and stir so the squash is thoroughly moistened. Season lightly with salt, cover, and cook, stirring occasionally and very carefully to avoid breaking up the squash, and adding water 1 tablespoon at a time to keep the squash from sticking to the bottom of the pan, until the squash is tender through, 15 to 18 minutes.