Hollow centers of zucchini by scooping out some of pulp; reserve.
Drop shells into boiling salted water; cook until just tender, about 4 minutes.
Drain.
Chop reserved pulp; set aside.
Cook onion about 5 minutes in hot oil in a skillet.
Remove from heat.
Add crumbs, garlic, parsley, lemon juice, salt, pepper, and egg yolk to onion along with chopped pulp; blend thoroughly.
Fill zucchini shells with mixture.
Dip in a mixture of 1 egg yolk beaten with 1 tablespoon olive oil.
Coat evenly with ./me dry breadcrumbs.
Fry zucchini in heated fat about 1 minute, or until lightly browned.
Fry only as many stuffed shells at one time as will float uncrowded one layer deep in fat.
Remove with slotted spoon and drain on absorbent paper.