Heat the olive oil in a large saucepan over medium heat. Add the onion and cook for around 10 minutes, stirring frequently until softened and lightly golden. Add the garlic and cook for a further minute.
Add the courgette, season with a little salt, stir well to coat in the oil, cover the pan and cook for 25 minutes, stirring occasionally, until the courgette is falling apart and is lightly coloured.
Add the vegetable stock and first portion of basil leaves and simmer for a further 10 minutes. Blitz in the pan using a stick blender until smooth. The soup can be made up to this stage one day in advance.
Whilst the soup is cooking, make the grissini. Preheat the oven to 240C/220C Fan/Gas Mark 9. Line 1-2 large baking trays with baking parchment. Combine the flour, Onken Natural Set Yogurt and sea salt in a large mixing bowl and bring together using a wooden spoon or your hands to form a dough. Take a small piece of dough (around 30g) and roll out on a lightly floured surface until long and thin. Place on the baking tray. Repeat with the remaining dough to get 2 long grissini per person. Bake in the oven for 12-15 minutes, until golden and firm. Leave to cool. These can be made up to two days in advance. Store in an airtight container.
To make the basil oil, place the olive oil and basil into a small blender and blend until fully combined. Strain through a small sieve into bowl. Cover the bowl with clingfilm and store in the fridge for up to 2 days.
When ready to serve the soup, re-heat the soup until hot. Remove the pan from the heat and whisk through the Onken Natural Set Yogurt. Season to taste. Divide the soup between shallow bowls. Swirl 1/2 tbsp of Onken Natural Set through each soup, drizzle with a little of the basil oil and garnish with a few basil leaves. Serve each soup with 2 breadsticks per person