Preheat the oven to 350 degrees. Place the pecans on a baking sheet and toast in the oven until golden and fragrant, 6 minutes; roughly chop.
Preheat a grill to medium-high heat on one side and medium heat on the other. Lightly brush the peaches with oil and grill over medium until nicely marked, about 4 minutes per side. Meanwhile, place a flat cast-iron skillet on the grill over medium-high heat. Add the bacon and cook until crisp; transfer to a paper-towel-lined plate.
Combine the spinach, red onion, grilled peaches, pecans and bacon in a large bowl.
Make the dressing: Mash the blue cheese with a fork in a bowl. Mix in the buttermilk, sour cream, sugar, and salt and pepper to taste. Toss with the salad just before serving.