Preheat the oven to 180°C. Place the bread on a baking tray and bake for 5 minutes or until golden brown and crisp. Rub the toast lightly with the cut garlic (discard the garlic). Cut the toast into cubes.
Meanwhile, heat 2 teaspoons of the oil in a large nonstick frying pan over moderate heat. Add the onion and cook for 4 minutes or until crisp-tender. Add the mushrooms and cook, stirring occasionally, for 4 minutes or until the mushrooms are softened. Transfer to a large bowl. Add the spinach and toasted bread cubes, tossing to combine.
Add the remaining oil to the pan along with the stock, vinegar and a pinch of salt. Bring to a rolling boil and cook for 1 minute. Pour the hot dressing over the spinach mixture and toss. Serve immediately.