Clean grill grates and spray with non-stick cooking spray. Pre-heat grill to high.

Add ground beef, sausage, Worcestershire sauce, onion, garlic, and rosemary to a large glass bowl. Season with salt and black pepper to taste, and combine thoroughly. Make sure both types of meat and all seasonings are completely incorporated throughout the mixture.

Divide the meat mixture into 8 equal parts and shape each part into ¾-inch thick patties. Place patties on a lined tray and use your thumb to create an indent in the middle to keep them from puffing up while cooking.

Place patties over direct, high heat on your grill and cook for 8-10 minutes. Actual cooking time will depend on the desired level of doneness, so check after 4-5 minutes and adjust the final cook time accordingly. (According to the USDA, the safe internal temperature for ground beef and pork is 160°F. Use an instant-read thermometer to confirm).

Flip and move burgers to indirect heat 2-3 minutes before they are done. Top with blue cheese and close the lid to allow the cheese to melt completely.

Transfer burgers to a platter and serve immediately on toasted hamburger buns and topped with lettuce, tomato, and avocado slices.

To toast buns, brush the inside surface of each bun with the melted butter and place over indirect heat while the burgers are cooking.