Grilled Eggplant Moussaka

Name: Grilled Eggplant Moussaka
Ingredients: 3 large eggplant, sliced into 1/4 inch rounds; 3 large potatoes, thinly sliced; 3 large zucchini, cut lengthwise into 1/4 inch slices; 0.5 cup extra-virgin olive oil; 5 tablespoons butter; 7 tablespoons all-purpose flour; 5 cups milk; 1 pinch ground nutmeg; salt to taste; 1 egg yolk, beaten; 1 tablespoon olive oil; 1.5 pounds ground beef; 1 onion, chopped; 1 teaspoon oregano; salt and pepper to taste; 0.5 cup chopped fresh parsley; 5 ripe tomatoes, chopped; 1 cup crumbled feta cheese