Preheat the oven to 180 degrees C. Bring a large saucepan of salted water to the boil. Add the zucchini and cook for 2 minutes. Remove from the water with a slotted spoon and drain well on paper towel. Add the eggplant slices to the water and cook for 2 minutes, then remove and drain. Add the potato slices to the water and cook for about 8 minutes until just tender. Drain and set aside with the other blanched vegetables.
Meanwhile, heat a cast-iron, ridged grill pan over a high heat. Cook the capsicum slices for about 5 minutes, turning occasionally, until slightly charred all over and starting to soften. Remove from the pan and roughly chop.
Combine all the cooked vegetables with the beans, chopped tomatoes and basil in a large baking dish. Drizzle the olive oil over the top, season to taste and stir together thoroughly.
In a bowl, stir together the yogurt, egg and parmesan until blended. Spread this mixture over the top of the vegetables in an even layer (it may not cover the vegetables completely, depending on the shape of the dish).
Bake the moussaka for 40 minutes or until the vegetables are tender and the topping is golden brown.