This is really Clams Casino, a fan-favorite appetizer, with walnuts as the driving force. The richness of the walnuts and the taste of the walnut skin really add amazing depth of flavor. These can be made in advance and baked off when needed. They can also be frozen and baked in a high heat oven when needed.
Place the butter in the bowl of a food processor and add the scallions, garlic, 3 teaspoons lemon juice and lemon zest. Pulse to blend. Transfer the mixture to a bowl; add the walnuts and season with salt and pepper. Add the breadcrumbs and stir to blend; set aside.
Heat ½ cup water in a large skillet until boiling. Place clams into the pan and cover with a lid. Let clams steam for 3 minutes or until some of the clams begin to open. Remove clams from pan and let sit until they are cool enough to handle. Pry the clam shells open using a butter knife, remove and discard the upper shell. Use a small knife to detach the clam body from the shell. Loosely mold some of the breadcrumb mixture into each of the shells so the clam body is somewhat covered. Arrange them in a single layer on a baking sheet. Bake for 10 minutes.
Preheat the broiler and broil them for a minute, watching them constantly so they don’t burn. Remove them from the oven. Drizzle with the remaining 1 teaspoon lemon juice. Top each clam with parsley. Serve immediately.