Preheat the oven to 350°. Wrap the garlic in a sheet of foil and bake for 1 1/2 hours, until tender. Let cool to room temperature.
Meanwhile, on a large baking sheet, drizzle the ciabatta pieces with olive oil. Bake for about 12 minutes, until crisp.
Peel the garlic. In a large skillet, heat 2 tablespoons of the olive oil. Add the onion and cook over moderate heat until softened, about 5 minutes. Add the apple cider and boil over high heat until reduced to 1/4 cup, about 4 minutes. Add the vinegar, roasted garlic and the remaining 2 tablespoons of olive oil and season with salt and pepper.
In a large bowl, toss the spinach with the croutons and warm dressing and serve.