Preheat oven to 400°. On a rimmed baking sheet, toss bread cubes with garlic, half of the grape tomatoes, and 3 Tbsp balsamic dressing. Sprinkle lightly with black pepper. Bake for 20 minutes, tossing halfway through. Remove sheet from oven and place on a cooling rack.
Meanwhile, heat a skillet over medium. Stack bacon and slice into six pieces. Place half of the bacon in the skillet and cook, stirring occasionally, until crispy, about 7 minutes. Using a slotted spoon, remove bacon pieces to a plate lined with paper towels. Drain bacon grease and repeat with remaining bacon.
In a large bowl, combine romaine lettuce, spinach, and mushrooms. Sprinkle with remaining 3 Tbsp Newman’s Own® Balsamic Vinaigrette dressing and toss to coat. Add the bread cubes to the lettuce and spinach and toss again. Divide salad among bowls and top with bacon pieces and cheddar cheese. Drizzle with additional dressing before serving.