For the croutons, cut bread into 1-inch cubes and place in a large bowl. Drizzle with olive oil and toss to coat. Sprinkle with seasoning and garlic. Arrange cubes in a single layer on a shallow baking sheet. Bake in preheated 400°F oven 15 to 20 minutes or until golden brown. Remove from oven and allow to cool.
Meanwhile, for the Blue Cheese Dressing, place all ingredients, except crumbled blue cheese, into a blender container or food processor. Blend until smooth. Add half of the crumbled blue cheese and pulse until smooth. Transfer dressing to a small bowl and gently stir in remaining crumbled blue cheese. Cover and refrigerate until ready to serve.
Sprinkle both sides of steak with Seasoning and massage into the meat. Grill over medium-high heat 6 to 8 minutes per side or until desired doneness. Let steak stand 5 minutes before cutting across the grain into thin slices.
Arrange lettuce on a large serving platter, topping with steak, red onion, avocado, tomatoes, croutons and remaining crumbled blue cheese. Serve with Blue Cheese Dressing.