Sauté vegetables: Heat 4 tablespoons butter and 1-2 tablespoons olive oil in a Dutch oven or heavy bottom skillet over high heat. As soon as it is melted and hot, add the 16 ounces fresh mushrooms and 1 large yellow onion. Stir, and allow to cook until liquid has evaporated and they are barely golden brown. Add 2 garlic cloves and 1-2 tablespoons onion dip mix, stir, and allow to cook for another 1-2 minutes. Remove from heat and set aside.
Sear and braise meat: Using the same skillet over high heat, cook the 1 1/2 pounds beef sirloin steak until brown. This should only take a few minutes. Add one cup of the beef broth and the 1 tablespoon Worcestershire sauce, reduce heat to low, cover, and simmer for 15 minutes.
Cook the noodles: Bring a large pot of salted water to a boil. Add the 16 ounces egg noodles and cook until they are al dente. Drain and keep warm.
Make the gravy: Whisk 1/4 cup all-purpose flour with the remaining cup of beef broth until smooth. Pour into pot with beef and stir well. Add the mushroom mixture back in with the beef and heat just until it starts to boil over high heat, stirring constantly. Allow the mixture to cook at a gentle simmer for about a minute.
Stir in sour cream: Reduce the heat to low and stir in the 1 1/2 cups sour cream. Heat the mixture until everything is heated through. Do not allow the cream to boil. Serve over noodles.