In a large bowl, combine the first 6 ingredients. Crumble beef and sausage over mixture. mix well. Shape into 8 patties. Grill, covered, over medium heat for 5 minutes on each side, or until well done. Top each burger with a cheese slice. Grill 1-2 minutes longer. Serve on buns with lettuce, tomato, avocado and mayonnaise or mustard. Makes 8 servings.