Preheat the oven to 350 degrees F. Soak the red onion in a small bowl of ice water, about 30 minutes. Drain and pat dry.
Meanwhile, make the croutons: Mash the anchovies with a fork. Combine the olive oil, mashed anchovies, 1 grated garlic clove and a pinch each of salt and pepper in a medium bowl. Let stand 5 minutes. Add the torn bread and toss with your hands until the bread absorbs the oil. Spread on a baking sheet and bake, stirring halfway through, until golden brown and crisp, 20 to 25 minutes. Let cool.
Whisk the buttermilk, sour cream, blue cheese, vinegar, parsley, the remaining grated garlic, 1/2 teaspoon salt and a few grinds of pepper in a large bowl until combined. Add the spinach, lettuce, cucumber, red onion and croutons; toss well.