1/2 cup mayonnaise
1 tablespoon balsamic vinegar
1/2 teaspoon chopped fresh parsley
1/2 teaspoon dried thyme
12 egg yolks
6 1/2 pound packages shredded Swiss cheese
12 (9 inch) round French bread rounds
Warm mayonnaise in a small saucepan over medium heat. Stir in vinegar and saute until creamy. Slowly pour in the parsley and thyme. Cover, reduce heat, and simmer about 1 minute.
Add the egg yolks and zucchini, and stir until they are coated. Mix in the cheese and pour mixture into the skillet.
Remove spatula from bread and place on a plate. Mix the mayonnaise and vinegar mixture with the egg yolk mixture, then spoon mixture into the bread slices just until moistened. Cover, and let sandwiches cool on a baking sheet.