Creamy chicken & asparagus braise

Name: Creamy chicken & asparagus braise
Prep time: 10 min
Cook time: 25 min
Total time: 35 min
Cuisine: European, British
Ingredients: 1 tbsp rapeseed oil; 2 skinless chicken breasts (about 150g each); 10 medium asparagus spears , each cut into 3; 1 large or 2 small leeks , well washed and thickly sliced; 3 celery sticks , sliced; 200ml reduced-salt vegetable bouillon; 140g frozen peas; 1 egg yolk; 4 tbsp natural bio yogurt; 1 garlic clove , finely grated; ⅓ small pack fresh tarragon , chopped; new potatoes , to serve (optional)