Step 1

Over low-medium heat, melt the butter in a saucepan and add the prepared onion, celery, and garlic. Stir in a pinch of salt and a generous grind of pepper.Cook, stirring often, until they're soft but not browned (10-15 minutes).

Step 2

Stir in the flour, cook for 2 minutes then take the saucepan off the heat.

Step 3

Combine the milk and stock then pour approximately 100ml into the saucepan. Stir well until it is combined with the other ingredients with no lumps then gradually stir in the rest of the liquid.Put the saucepan back on the heat, turned up to medium-high. Add another pinch of salt and a little more pepper. Stir constantly while the soup comes up to a simmer. Turn the heat down to low and simmer for 5 minutes.Tip: make the cheese toasts while the soup is simmering.

Step 4

Take the soup off the heat, add the grated cheese, then stir until it melts. Check the seasoning again.Serve the soup into warm bowls, topping each with 3 or 4 cheese toasts.

Step 5

Cut the bread into 12-16 squares then toast on one or both sides, as preferred.Sprinkle with the grated cheese (on the untoasted side if applicable) then put under the grill until the cheese is bubbling and golden.