Preparation

• Position a rack in the center of the oven, and heat the oven to 400°F. Trim both ends of the zucchini, and cut each one in half lengthwise. Use a grapefruit spoon to scoop out the seeds and most of the flesh, leaving the remaining zucchini shell about 1/4 inch thick. Coarsely chop the scooped-out seeds and flesh, and set aside.
• Heat 3 Tbs. of the oil in a large Dutch oven or large sauté pan over medium heat. Cook the onion, stirring occasionally, until beginning to soften, about 2 minutes. Add the garlic and chard stems, and cook, stirring frequently, until the garlic is fragrant, about 1 minute. Add the zucchini flesh and cook, stirring frequently, until the onion and zucchini fully soften and any liquid cooks off, 4 to 5 minutes. Stir in the chard leaves, 1/2 tsp. salt, 1/4 tsp. pepper, Italian seasoning, 1/4 tsp. of the fennel seeds, and the nutmeg. Reduce the heat to medium low, and cook until the chard is tender and any remaining liquid in the pan has cooked off, another 3 to 5 minutes.
• Transfer the mixture to the bowl of a food processor, and pulse until the mixture is well combined. Do not overprocess; the mixture should have a coarse texture with some smoothness. Transfer to a large bowl, and stir in the Parmigiano-Reggiano and about half of the basil. Season to taste with salt and pepper. Stir in the egg and then 1/2 cup of the panko.
• In a medium bowl, combine the remaining 3/4 cup panko, 1 Tbs. plus 1 tsp. oil, 1/4 tsp. salt, and 1/4 tsp. fennel seeds.
• Brush a large baking dish with 1 Tbs. of the oil, then brush the cut side of each zucchini lightly with oil, about 2 tsp. divided. Season each zucchini half with a pinch of salt and pepper. Use a small spoon to fill each zucchini with the chard mixture, distributing it evenly among the zucchini halves. Top each zucchini with 2 generous spoonfuls of the panko crumb mixture, about 1-1/2 heaping Tbs. on each zucchini half. Transfer the stuffed zucchini to the baking dish and then to the oven. Bake until the zucchini are tender and the panko topping is golden-brown, about 25 minutes.
• Meanwhile, combine the ricotta, the remaining 1 Tbs. oil, salt to taste, and lemon zest in an oven-safe bowl or small baking dish. Bake until just hot, about 7 minutes.
• Transfer the zucchini to a serving dish, and top each half with a spoonful of ricotta, the remaining basil, and the chopped parsley, if you like.