Preheat oven to 350°.
Season the eggplant slices with salt and pepper.
Heat a 12-inch oven-safe skillet, or a 3-quart Dutch oven, over medium-high heat and add 2 tablespoons of the olive oil. When the oil shimmers, add half of the eggplant slices and cook until limp and lightly browned on both sides, about 5 minutes. Transfer the eggplant to a plate, add the remaining 2 tablespoons of olive oil to the pan and cook the remaining eggplant in the same way.
Add the onion, zucchini and 1/4 teaspoon of salt to the hot skillet and sauté until the vegetables soften, about 4 minutes. Add the garlic, tomato, vinegar and cinnamon and cook until the tomato gives off its juices, about 4 minutes. Taste and season with more salt and pepper, if needed. Spread the vegetables evenly in the pan, arrange the eggplant slices on top and sprinkle with the kefalotyri.
Transfer the pan to the oven and bake until the top is crispy and the flavors have blended, about 15 minutes.
Arrange the bread on heated plates, spoon the moussaka over the bread and serve hot or at room temperature.
From One Pan, Two Plates: Vegetarian Suppers by Carla Snyder; photographs by Jody Horton (Chronicle Books, 2016).