Heat the grill to medium. Rinse the spinach, remove any tough stems and drain well.
To make the croutons, cut the bread into small dice. Mix the garlic purée or crushed garlic and Tabasco sauce together in a bowl, then stir in the bread. Transfer the spiced bread to a baking sheet or grill pan in a single layer and grill for 1-2 minutes, turning the dice over occasionally, until the croutons are golden brown. Set aside.
Remove a few strips of lemon zest and reserve, then squeeze the juice. Put the lemon juice, mushrooms, soy sauce and green onions in a saucepan over a medium-high heat and cook, shaking the pan occasionally, for 2-3 minutes until the vegetables have softened.
Turn the heat to high, add the spinach and stir for 2 minutes, or until the spinach has wilted and most of the juices have evaporated. Season with pepper. Serve the spinach hot, with the croutons and reserved lemon zest scattered over the top.