Season the chicken breasts with salt as required and reserve
In a large pan or a braiser, add a glug of oil over medium-high heat and sear the chicken breasts on one side till they are golden brown. Set aside
To the same pan, add the butter, onions, and mushrooms, and sauté with a pinch of salt on low heat for 10 minutes. Add the rosemary leaves and give it a quick stir. Then add the white wine and let it bubble away for a couple of minutes
Once the wine has almost evaporated, add ½ a pouch of the Romesco, the crushed tomatoes, and the stock and bring to a simmer
Once the mixture is simmering, nestle the chicken (seared side up) back into the sauce
Cook for about 6-8 minutes till the chicken breast is fully cooked
Garnish with fresh parsley and serve immediately