Melt the butter, but make sure it doesn’t turn brown. Simmer the vegetable mix with the sweetcorn until the vegetables are fully defrosted and glassy. Add water and stock and bring to the boil. Mix the soup until smooth. If required, sieve the soup to remove any skin of the sweetcorn. Season with salt, pepper and the lemon balm. Chef's tip: “Less skin in the soup? Use our babycorn cobs.”