Asparagus tempura with herb and garlic sauce

Name: Asparagus tempura with herb and garlic sauce
Cuisine: Japanese
Ingredients: 2 litres flavourless oil (such as sunflower) for deep-frying; 50g plain flour; 50g cornflour; 175ml ice-cold fresh soda water; 800g thin asparagus spears; 2 tbsp soured cream or crème fraîche; 4 tbsp good-quality olive oil mayonnaise; 2 tbsp full-fat natural yogurt or; Greek yogurt; 1-2 small garlic cloves, crushed; 25g mixed fresh herbs such as tarragon, chives, basil and parsley,; plus extra chopped herbs to serve; 1 tsp lemon juice; Digital probe thermometer