In a small bowl, combine crème fraîche, blue cheese, garlic, chives and salt and pepper. Whisk well.
Cut croissants in half (if not already done). Arrange lettuce on each bottom half of the croissants. Top with deli roast beef, about 2-3 slices each, depending on the thickness.
Top beef with tomato slices and a generous spoonful of blue cheese aioli.