Peel and finely chop the onion. Rinse and slice the mushrooms. Rinse and halve the cherry tomatoes. Halve the cabbage. Finely shred one of the halves. Rinse well.
Peel and grate or crush the garlic. Pull the leaves off half the rosemary sprigs. Rinse them. Finely chop. Put a handful to one side. Fill and boil the kettle.
Warm a glug olive oil in a large frying pan or wok. Add the chicken. Fry for 5 mins till browned. Stir every so often so it browns evenly.
Add the onion and mushrooms to the chicken. Season with salt and pepper. Fry for 8-10 mins over a medium heat till the veg is soft and lightly golden. Stir often while they cook.
Fill a pan with hot water from the kettle. Bring to the boil. Add the spaghetti. Pop on the lid. Gently boil for 5 mins till the spaghetti is almost cooked.
Take the lid off the spaghetti. Add the shredded cabbage. Simmer, without the lid, for 5 mins till the spaghetti and cabbage are tender but not too soft.
Whilst the spaghetti cooks, add the garlic and rosemary to the chicken. Stir and fry for 1 min. Add the tomatoes. Fry for 2-3 mins to soften them. Add the tub of crème fraiche. Stir over a low heat for 2-3 mins to heat everything through. Taste and adjust the seasoning if needed.
Drain the spaghetti and cabbage. Divide between two warm plates. Top with the creamy chicken and rosemary sauce. Garnish with the chopped rosemary you saved in step 2.