Preheat the oven to 350°F.
Wrap the garlic in a tin foil packet, drizzle with olive oil, close the foil and roast until very tender and fragrant, about 20 minutes.
Transfer the garlic cloves into a blender. Add the olive oil, mayonnaise, vinegar, sour cream, salt and pepper. Blend until smooth, adding water if necessary to reach desired consistency. Fold in the blue cheese crumbles, taste and adjust seasoning.
Tear or slice the bread into cubes. Toss the bread cubes with the melted butter on a baking sheet. Season with salt. Roast until golden brown, about 10-12 minutes. Cool.
Heat a skillet over medium heat. When hot, add the bacon and cook, turning as necessary, until crisp. Transfer to a paper towel-lined plate. When cool, crumble into small pieces.
In a serving bowl, toss together the lettuce, bacon, onion, tomatoes, blue cheese, croutons, chives and salad dressing.