Add the oil and butter to a heavy based saucepan and set over a low heat. Once the butter has melted, add the onions, thyme and sugar. Allow to fry very gently, stirring occasionally, until they are soft and caramelised. This will take at least half an hour, perhaps a little more but be patient, the caramelisation will add masses of flavour to the soup.
Pour in the cider, turn up the heat a little and allow it to bubble away and evaporate for a few minutes before pouring in the stock. Season well with salt and pepper, bring up to the boil and simmer steadily for 20 minutes. Taste to check the seasoning and keep warm.
To make the cheddar toastie croutons, toast the bread on both sides under the grill. Spread each with mustard to taste and sprinkle on the cheddar. Return to the grill for a minute or two until the cheese is melted and bubbling. Allow to cool for a moment before chopping into 1cm squares.
To serve, ladle the soup into deep bowls and scatter the croutons on top. Eat immediately.
