Peel and finely chop the onion. Trim the carrot and peel it. Finely chop it into small dice. Trim the dry ends from the celery stick and chop it into small dice to match the carrot. Trim the ends from the courgette and finely dice it as well.
Warm a large pan for 1 min. Add ½ tbsp olive oil and the diced chicken leg. Fry for 5-6 mins, stirring now and then, to brown it all over.
Add the diced veg to the pan. Season well with salt and pepper. Fry over a medium-low heat for 5 mins, stirring often, till the veg are tender, and have a bit of colour.
Finely chop all but a few of the rosemary leaves while the veg cook. Stir them into the pan. Crumble in the stock cube.
Pour in the jar of passata. Add 400ml boiling water. Pop on a lid. Turn up the heat and bring to the boil. Turn the heat down and simmer for 5 mins to mingle the flavours.
While the soup simmers, break the spaghetti into small pieces. Stir the spaghetti into the pan. Put the lid back on and simmer for 5 mins. (The spaghetti will be almost cooked, but not quite.)
Tear the spring green leaves from their thick cores and roughly chop them. Add the spring greens to the pan. Simmer, without the lid on, for 5 mins to finish cooking the spaghetti and to cook the spring greens.
Taste the minestrone, and add more salt and pepper if you think it needs it. Ladle into warm bowls and sprinkle with the last of the rosemary leaves to serve.