Start by adding half of the oil into a non-stick frying pan. Increase the heat to high and add the mushrooms into the pan. Distribute across the pan, then leave to brown for 5 minutes.
Once the mushrooms have taken on some colour, stir and continue to cook for around 2 minutes.
Remove the mushrooms from the pan and set aside. Reduce the heat to low/medium and add the remaining oil. Add the onion and chicken to the pan and season with salt and pepper. During this time, add your pasta shells to heavily salted boiling water, setting your timer to the packet time minus 2 minutes.
Fry for around 4 minutes, then add the garlic and rosemary. Continue to fry for around 3 minutes, then add the cream into the pan.
Once the cream is up to room temperature, slowly pour the stock into the pan, along with the cornflour slurry (water & cornflour), Worcester sauce and half of the mushrooms. Continue to cook until thickened and season with salt and pepper.
Reserve a mug full of pasta water, drain the pasta and add to the pan. Stir the pasta until fully coated in the creamy sauce, loosening with pasta water to form a glossy, creamy sauce.
Top with the reserved mushrooms and a crack of black pepper.