- Combine all the ingredients with a whisk. Season with salt and pepper. Set aside.
- With the rack in the middle position, preheat the oven to 200 °C (400 °F).
- On a baking sheet, toss the onions with half the oil. Season with salt and pepper. Bake, stirring frequently, for 35 to 40 minutes or until the onions are tender and begin to caramelize.
- Meanwhile, in a skillet, brown the bread in the remaining oil. Season with salt. Drain on a paper towel.
- Divide the spinach among four plates. Garnish with the roasted onions and croutons. Drizzle with the vinaigrette.