Preheat the oven to 450 degrees Fahrenheit.

Trim off the ends of each zucchini.

Cut each zucchini in half crosswise.

Cut each zucchini piece in half lengthwise.

Scoop out the centers of the zucchini, leaving a shell at least 1/4-inch thick.

Reserve and finely chop the scooped-out zucchini portions.

Pour water into a saucepan to cover the zucchini shells and bring to a boil.

Add the zucchini shells to the boiling water and simmer for 2 minutes.

Drain the zucchini shells thoroughly.

Clean and finely chop the fresh mushrooms.

Heat the butter in a heavy skillet over medium heat.

Add the finely chopped onions and minced garlic to the skillet and cook briefly, stirring.

Add the chopped mushrooms and lemon juice to the skillet.

Stir and cook for about 1 minute.

Add the chopped zucchini, fresh thyme, salt, and pepper to the skillet.

Cook for about 3 minutes until the excess moisture evaporates.

Spoon and scrape the mushroom mixture into a mixing bowl and let cool slightly.

Add 1 cup of the fine fresh bread crumbs, the finely chopped parsley, and 2 tablespoons of the grated Parmesan cheese to the bowl. Blend thoroughly.

Stir in the egg yolk.

Brush the inside of a baking dish with 1 tablespoon of olive oil.

Spoon an equal portion of the filling into each zucchini shell and smooth over the top.

Blend the remaining 2 tablespoons bread crumbs with the remaining 2 tablespoons cheese.

Sprinkle the bread crumb mixture evenly over the top of the stuffed zucchini boats.

Sprinkle evenly with the remaining 2 tablespoons olive oil.

Place the dish in the preheated oven and bake for 25 minutes.

Serve the stuffed zucchini boats immediately.