For the dressing: Combine the creme fraiche, buttermilk, basil, parsley, chives, thyme, garlic, lemon juice and onion powder with a whisk in a medium bowl. Fold in the blue cheese, season with salt and pepper and refrigerate until ready to serve.
For the wedge: Preheat the oven to 350 degrees F. Spread the pecans on a rimmed baking sheet and toast until fragrant, 6 to 8 minutes. Let cool and then chop.
Divide the lettuce wedges among 4 plates. Top each with 1/4 cup of the dressing, the tomatoes and toasted pecans. Serve the remaining dressing on the side. Eat right away!