Cut the zucchini in half lengthwise, then scoop out the flesh with a teaspoon or a tablespoon, depending on the zucchini size. Leave the shell at least 6-7 mm (¼") thick, even more, if it's a very large zucchini. Keep about ⅓ of the zucchini flesh aside.
Sprinkle the zucchini shell with salt and place it cut side down on a paper towel for a few minutes to remove the excess water.
Meanwhile, prepare the stuffing. In a bowl, mix the canned tuna with the minced onion and ⅓ of zucchini flesh you set aside, chopped finely.
Add the egg, lemon juice, and breadcrumbs and mix well. Season with salt and pepper.
With a tablespoon, stuff the zucchini, pressing well to make sure there are no air pockets.
Place the zucchini on a baking sheet, then sprinkle on top with chopped parsley, more breadcrumbs, and drizzle with olive oil.
Bake for about 20-25 minutes or until a fork can easily pierce the zucchini.
Notes
Refrigerate leftovers in an airtight container for up to 2 days. Reheat in the oven at 350°F until warmed through, preserving the texture and flavor best.