Heat olive oil in a frypan; cook and stir the garlic in the hot oil until slightly browned, 2 to 3 minutes. Sprinkle in the rosemary and let cook until fragrant, 1 to 2 more minutes. Cook and stir the silverbeet until the leaves are wilted; cover the frypan and let the silverbeet steam until the stems are tender, about 5 minutes. Transfer the silverbeet mixture to a bowl, stir in the tarragon and basil, season to taste with salt and pepper and let cool. Stir in the mozzarella.
Trim the stem and blossom ends from the zucchini and halve lengthwise. Scoop out the flesh with a spoon, leaving a 1-2cm thick shell. Discard the flesh. Beat 2 eggs with the milk in a bowl. Lightly fill the shells with the silverbeet mixture. Pour the egg mixture over the silverbeet.
Bake in the preheated oven until the filling is set and the zucchini shells are tender, about 40 minutes.
To prepare crumb topping, melt butter in a frypan over medium heat; cook and stir the bread crumbs in the butter until well combined and starting to give off a toasty fragrance, about 2 minutes. Remove the stuffed zucchini from the oven and gently pat the bread crumbs over the top in a layer; return to oven and bake until the crumbs are browned, 10 to 15 more minutes.