Step 1

Combine shallot, vinegar, chile flakes, ½ tsp. sugar, and ½ tsp. salt in a small nonreactive bowl; let sit at room temperature until ready to use.

Step 2

Meanwhile, cook bacon in a large skillet over medium heat, stirring often, until fat has rendered and bacon is browned and crisp, 8–10 minutes. Transfer bacon to paper towels. Pour off 1 Tbsp. fat; set aside for dressing.

Step 3

Return skillet to medium heat and add mushrooms and garlic to remaining bacon fat; season with salt and pepper. Cook, stirring occasionally, until mushrooms are golden brown, 6–8 minutes.

Step 4

Whisk buttermilk, sour cream, mayonnaise, lemon juice, garlic powder, onion powder, and reserved bacon fat in a medium bowl until smooth. Season with salt and pepper. Stir in herbs; cover and chill until cold, at least 30 minutes.

Step 5

Transfer shallot to a large bowl; discard vinegar mixture. Add spinach, dressing, bacon, and mushrooms and toss to coat. Divide salad among plates.

Step 6

Do Ahead: Dressing can be made 1 day ahead. Cover and chill.