Put a large pan of water on to boil. Heat your oven to 180°C/Fan 160°C/Gas 4. When the water is boiling, add the pasta. Simmer for 8 mins, then drain.
While the pasta is cooking, warm a large, deep frying pan for 1 min. Add 2 tsp olive oil and the chicken leg. Fry over a medium heat, turning once or twice, for 8-10 mins till golden all over.
While the chicken is frying, trim the ends off the courgette then slice it into thick slices. Chop these into small dice. Trim the roots off the spring onions and finely chop them. Finely chop most of the tarragon leaves.
Lift the chicken out of the pan and pop on a plate. Add the courgette and spring onions to the pan with a splash of water. Season with a pinch of salt and pepper. Cook and stir for 5 mins to soften the veg. If the pan dries out and they start to catch, add a splash more water.
Return the chicken to the pan. Stir in the tarragon, cream and mustard. Take off the heat. Stir in the drained pasta. Taste and add more salt and pepper if you think it needs it.
Spoon the pasta and sauce into an ovenproof dish. Sprinkle over the breadcrumbs. Bake for 20 mins till the pasta bake is golden and bubbling.
Toss the salad leaves with 2 tsp olive oil and some salt and pepper. Serve the pasta bake in warm bowls with the salad on the side.