Cook the tagliatelle in a large pan of boiling water, according to the pack instructions, drain and set aside.
Heat the oil, in a large sauté pan and fry the onion and garlic for 2-3 mins.
Add the diced turkey breast, and cook until browned on all sides.
Stir in the mushrooms, and cook for a further minute or two, then add the soft cheese, lemon juice and zest, milk and seasoning.
Stir to combine then warm the sauce but do not let it boil.
Add the tarragon and stir well before serving with the cooked pasta.