For the salad: Preheat the oven to 400 degrees F.
Line a rimmed baking sheet with parchment paper. Set a rack on the baking sheet and lay the bacon on it in a single layer. Cook until crisp, 35 to 45 minutes, turning halfway through. When cool enough to handle, chop coarsely.
For the ranch dressing: In a bowl or a jar with a tight-fitting lid, add the buttermilk, sour cream, chives, mayonnaise, parsley, lemon juice, salt, celery seed, onion powder, garlic powder, garlic and scallions and whisk together (or shake vigorously if using a jar).
Assembly: Lay a lettuce wedge on each of 6 salad plates. Drizzle each wedge with 2 to 3 tablespoons of the dressing, letting it flow between the leaves of the lettuce and pool on the plate. Sprinkle over the tomatoes, onion and bacon. Garnish with the chives.
Any leftover dressing can be refrigerated for up to 1 week.
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