Herring tacos

Name: Herring tacos
Cook time: 20 min
Total time: 20 min
Cuisine: American, Mexican, Latin American
Ingredients: 8 herring fillets; 4/8 small soft flour tortillas; 2 tbsp fajita seasoning; 1 red onion; 100g butter; 2 tbsp white wine vinegar; 1 little gem lettuce shredded; 1 large ripe avocado; 2 limes; handful of cherry tomatoes; 1 red chilli; bunch of coriander; 150ml natural yogurt