Tip all of the ingredients for the gazpacho into a large bowl and mix well. Cover and chill for 4-5 hours or overnight.
Use a stick blender or food processor to whizz the gazpacho mixture until very smooth, adding more salt or sherry vinegar to taste. Chill.
Heat the oven to 200C/fan 180C/gas 6. Put the sourdough croutons and garlic onto a baking tray, drizzle with the oil, season and toss well. Roast for 12-15 minutes or until really crisp. Add the pesto to the tray and toss really well again, then sprinkle over the manchego and cook until crisp and bubbling.
Spoon the gazpacho into bowls, then top with the cheesy croutons and a drizzle of oil.