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Heat the oil in a large non-stick frying pan and fry the chicken for 5 mins, turning to brown both sides.
Add the asparagus (reserve the tips), leeks and celery, pour in the bouillon and simmer for 10 mins. Add the asparagus tips and peas, and cook for 5 mins more.
Meanwhile, stir the egg yolk with the yogurt and garlic. Stir the yogurt mixture into the vegetables and add the tarragon. Divide between two warm plates, then place the chicken on top of the vegetables. Serve with new potatoes, if you like.