Arrange a rack in the middle of the oven and preheat the oven to 375◦F. Lightly oil a small, parchment-lined baking dish large enough to snugly fit the zucchini. Set aside.
Prep all ingredients according to specifications above. If using long zucchini, trim the ends, then cut them in half lengthwise. With either a teaspoon or grapefruit spoon, gently hollow out the zucchini halves by removing the soft inner flesh, making sure to leave the heartier zucchini base in place to be stuffed. This is best done by scraping the spoon towards you along the length of the zucchini while holding it vertically. If using round zucchini, cut off the top stem, then either cut the round bulb in half and scoop out the flesh as described above for the long zucchini, or leave the zucchini bulb whole and scoop out all flesh from above. Rough chop the flesh and set it and the hollowed out zucchini aside.
Heat the olive oil in a large sauté pan or Dutch oven over medium heat, then add the onions and garlic and cook until the onions have mostly softened, about 4 to 5 minutes.
Add the reserved zucchini flesh and continue to cook 4 to 5 minutes or until all residual liquid has evaporated.
Turn the heat off, then fold in the sun-dried tomatoes and basil, followed by the breadcrumbs and lemon zest. Season to taste with salt and black pepper.
Carefully fill each dug-out zucchini shell with the zucchini and onion mixture, then nestle the stuffed vegetables in the roasting pan. Top each zucchini with a light sprinkle of additional breadcrumbs and a drizzle of olive oil.
Pour enough water in the roasting pan to fill it about ¼”. Cover the pan with foil, then place it on the middle rack in the oven. Roast for 30 to 35 minutes or until the flesh of the zucchini base is tender when pierced with the tip of a knife. Then, uncover and roast an additional 5 to 10 more minutes to brown the Stuffed Zucchini, taking care to not burn them. (NOTE: If the zucchini are large, they may take longer to cook and require additional liquid in the pan. Add more water if necessary so that the baking dish does not dry up and the zucchini start to fry and burn.)
Remove from oven and transfer to a serving platter. The Stuffed Zucchini can be served either hot or at room temperature. Buon Appetito!