These can be assembled a day ahead and refrigerated. Remove them from the refrigerator 30 minutes before baking.
- 1 teaspoon extra-virgin olive oil, plus more for baking dish
- 4 medium zucchini, halved lengthwise (about 2 pounds)
- 1 pound spicy Italian sausage, removed from casing
- 1 small onion, coarsely chopped (about 1 cup)
- 3 cloves garlic, minced (about 2 tablespoons)
- 1 cup coarse fresh breadcrumbs
- 2 teaspoons lemon zest, plus 1 tablespoon juice (from 1 lemon)
- 1 1/2 ounces grated Parmesan (about 3/4 cup), plus shavings for garnish
- 1 teaspoon coarse salt
- 1 large egg, lightly beaten
Preheat oven to 350 degrees. Drizzle a large baking dish with oil. Scrape seeds and soft flesh from zucchini and reserve 1/2 cup. Place zucchini, cut sides up, in baking dish.
Heat oil in a large skillet over medium-high heat; add sausage and onion. Cook, breaking up sausage, until sausage is browned and onion is softened, about 5 minutes. Add garlic and reserved zucchini; cook until garlic is fragrant, about 1 minute. Remove from heat; stir in remaining ingredients. Divide mixture among zucchini. Bake until filling is firm and each zucchini is tender and wrinkled around edges, 35 to 40 minutes. Garnish with Parmesan shavings.