Preheat your oven to 180°C (350°F). Slice off the stems of each zucchini, then cut them in half lengthwise.
Using a spoon, carefully scoop out the flesh from each zucchini half, leaving about 1/4 inch border to keep the shell intact. Reserve the flesh for the filling and set the shells in your baking dish.
Chop the reserved zucchini flesh into small pieces. In a skillet over medium heat, warm the olive oil until it shimmers and begins to smell fragrant.
Add finely chopped onion and minced garlic to the skillet, sautéing until golden and fragrant, about 3-4 minutes. The onions should be translucent and the garlic aromatic.
Stir in the chopped zucchini flesh and cook until tender, about 5 minutes, allowing the flavors to meld and the zucchini to soften. Season with salt, pepper, and a squeeze of lemon juice.
Transfer the sautéed mixture to a mixing bowl. Add shredded mozzarella, grated Parmesan, chopped herbs, and toasted breadcrumbs. Mix until well combined.
Stuff each zucchini shell with the filling, pressing gently to pack it in. Place the stuffed zucchinis in your prepared baking dish, and sprinkle a little extra cheese on top if desired.
Bake in the preheated oven for 25-30 minutes, until the zucchini edges are caramelized and the filling is bubbly and golden on top.
Remove from the oven, let rest for 5 minutes, then garnish with fresh herbs or a drizzle of olive oil before serving. The zucchini should be tender and the filling rich and gooey, with a tempting golden crust.
