Heat olive oil in a large sauce pan over medium-low heat. Add the onions, celery, garlic, salt and pepper, cover with a lid to sweat. Stir every now and again, until vegetables are soft and just going golden often until golden brown, about 20-30 minutes.
Add the cashews, sugar, broad beans and water, bring the mixture to a simmer covered. Cook until the vegetables are soft and loosing their structure, 10 to 15 minutes.
With a stick blender, blend the soup until as smooth as possible (or put in a blender and blend in batches). Press the puree through a fine-mesh sieve into a medium saucepan; discard the solids. Thin the soup with water until you get a velvety, pourable consistency. Stir the malt vinegar into the soup. Adjust seasoning with more salt and pepper if needed and warm over medium-low until heated through.
Serve with a scattering of spring onions, dribble of olive oil, sprinkling of Chile Arbol Flakes and fried salt and vinegar corn tortillas croutons on top.
To make the croutons:
Heat the oil to fry the corn tortillas. When the oil has reached 180oC, drop in a few of the tortillas and fry for 1 minute until just golden (they will continue to get darker as they cool). Remove from the oil and drain on paper and whilst still warm dust with malt vinegar powder and sprinkle with sea salt. Continue frying in small batched until all the tortillas are cooked. Store in a airtight container until you are ready to serve.