Heat the butter in a pan and, when foaming, gently fry the onion and garlic for 3-4 minutes. Pour in the stock, bring to the boil, reduce the heat and simmer for 6-8 minutes.
Stir in the spinach and conmtinue to cook until wilted.
Using a hand held blender, blend until smooth and season, to taste with salt and freshly ground black pepper.
For the croutons, place the bread on a baking tray and bake in the oven for 3-4 minutes.
Top with the cheese and place back in the oven for a further 4-5 minutes until the cheese is bubbling and golden-brown. Remove and cut each slice in half.
Serve the soup in a bowl with the Gruyère croutons on the side.
