How to Make It
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.
The ciabatta croutons can be kept in an airtight container at room temperature overnight. The roasted garlic can be refrigerated overnight; bring to room temperature before using.