If using zucchini and Japanese eggplant, trim 2 inches off one end. Dice the trimmed part and reserve for the filling. Cut a small slit lengthwise down the center of the eggplant and place it on a baking pan along with the zucchini. Drizzle with olive oil and season with salt and pepper and roast the prepared zucchini and eggplant covered for 30 minutes at 400 degrees.
Par-boil the rice for 10 minutes in boiling water.
Start by cutting the tops of the peppers and tomatoes and scoop out the insides to hollow out the veggies. Finely chop the tomato insides and place them into a bowl for the filing.
Once all the veggies are chopped and your peppers and tomatoes are hollowed out, you can prepare the filling.
In a large pan, heat 4 tablespoons olive oil, and saute the onions, leeks, and green onions until tender, about 4-5 minutes. Add diced celery, chopped peppers, diced mushrooms, diced zucchini, eggplant, and chopped tomatoes. Cook until tender, about 8 minutes.
Add the minced garlic and cook until fragrant, one minute. Then, stir in the par-boiled rice and add the chopped mint, and parsley, and season with salt, pepper, and dried oregano.
In the skillet with the veggies, pour 2 cups tomato sauce (pasta sauce) and 1 cup vegetable broth. Cook for 10 minutes over medium heat.
Meanwhile, remove the zucchini and eggplant from the oven and cool until you can handle them.
Using a small knife cut the top of the zucchini like a boat shape so that you can hollow it out and stuff it. Hollow out both the zucchini and remove the seeds and insides of the eggplant.
Place all the hollowed veggies into a baking dish and add the filling mixture. Scatter potatoes around the baking dish to fill any open space and help the peppers sit upright.
Replace the tops of the tomatoes, peppers, and zucchini. Drizzle generously with olive oil, and season with salt, pepper, and dried oregano.
In a measuring cup, mix together 1 cup of tomato sauce (pasta sauce) with ½ cup of vegetable broth. Pour this mixture over the stuffed vegetables and potatoes.
Cover the pan with foil and bake the Gemista for 1 hour at 400 degrees. Remove the foil for the last 20 minutes of cooking for the veggies to get some color.