Preheat the oven to 425°F. Line a baking sheet with parchment paper.
Cut each zucchini in half lengthwise. Use a spoon or a grapefruit spoon to scoop out the seeds and most of the flesh, leaving the remaining zucchini shell about ¼ inch thick. Arrange the zucchini, cut-side up on the prepared baking sheet. Coarsely chop the scooped-out flesh, and set aside.
Heat 1 Tbs. of the oil in a large nonstick skillet over medium heat. Add the pancetta and cook, stirring occasionally, until golden and starting to crisp, about 3 minutes. Add the onion, stirring occasionally, until beginning to soften, about 2 minutes. Add the garlic and cook, stirring frequently, until fragrant, about 1 minute. Add the zucchini flesh and cook, stirring frequently, until softened and any liquid cooks off, 4 to 5 minutes. Stir in the pine nuts, lemon zest, herbs, ½ tsp. salt, ¼ tsp. pepper, and the nutmeg. Remove from the heat and stir in ½ cup of the parmesan.
In a small bowl, combine the panko crumbs and the remaining ½ cup of Parmesan.
Brush the cut side of each zucchini lightly with oil. Season each zucchini half with a pinch of salt and pepper. Use a small spoon to fill each zucchini with the pancetta mixture, distributing it evenly among the zucchini halves. Top each zucchini with panko crumb mixture, about 1-½ heaping Tbs. on each zucchini half. Drizzle each with the remaining olive oil.
Bake until the zucchini are tender and the panko topping is golden-brown, about 25 minutes. Serve with warm marinara, if desired.