Fill a large saucepan with water, add 1 tablespoon of salt and bring to the boil.
Add 340g gemelli pasta and cook for 8-10 minutes until al dente. Reserve about 240ml of the cooking water, then drain the pasta.
Heat 1 tablespoon of olive oil in a large frying pan over medium-high heat. Add the chicken pieces seasoned with 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Cook for 5-7 minutes until no longer pink, then transfer to a plate.
Reduce the heat to medium. Add 1 tablespoon of olive oil to the same pan and sauté the onion for 3-4 minutes until softened.
Stir in the garlic and oregano, cooking for 1 minute until fragrant.
Pour in the tinned crushed tomatoes and chicken stock, scraping up any browned bits from the base of the pan. Bring to a gentle simmer, then reduce the heat to low, cover and cook for 10 minutes.
Stir in the double cream, then add the spinach in batches, stirring until wilted.
Return the chicken to the sauce. Add the drained pasta and stir through to combine. If the sauce is too thick, loosen with a little of the reserved pasta water, adding a tablespoon at a time until you reach the desired consistency.
Serve immediately, topped with grated Parmesan and fresh basil leaves, if desired.