Heat oil in a large saucepan, add chillies, onion, garlic, salt and pepper and cook over low heat for about 10 minutes, until onion is soft.
Add sweet potatoes and stock, bring to a simmer and cook uncovered for about 20 minutes, until sweet potato is soft.
Process the mixture in a food processor, or with a hand blender, until smooth.
Stir in lime juice, check seasoning and adjust as necessary.
Cut tortillas into strips about 4cm long and 1cm wide. Deep fry in batches in hot vegetable oil until golden and crisp. Drain on absorbent paper.
Serve the soup with a spoonful of sour cream and garnished with coriander and tortilla crisps.