Preheat your oven to 350 degrees.
Cook the pasta according to package directions. Meanwhile, melt 2 tablespoons of butter in a skillet & saute the onion for about 5-6 minutes on medium high heat. Drain the pasta.
In a large bowl, combine the 2 cans of clam chowder with the beaten eggs, Worcestershire sauce, lemon juice, 2 teaspoons of Italian dressing mix, garlic powder, & black pepper to taste. Stir in the cooked pasta, sauteed onions, & shredded cheese. Spread the mixture into a 9x13 casserole dish.
In a separate bowl, combine 4 tablespoons of melted butter with the remainder of the Italian dressing dry mix. Fold in the oyster crackers. Spread the seasoned crackers over the pasta. Sprinkle with some diced green onions, reserving some to sprinkle on after the casserole has baked.
Bake for 30 minutes. Sprinkle with remainder of diced green onions. Let cool for 5 minutes before serving.