Bring a large pot of salted water to a boil and cook linguine until al dente; reserve a little pasta water before draining.
Toss chicken with cornflour, salt, pepper, and garlic powder until coated.
Heat a pan, melt butter, and cook chicken until golden and fully cooked; set aside.
In the same pan, sauté onion and garlic until soft and fragrant.
Pour in chicken stock and heavy cream; simmer until lightly thickened.
Add linguine and parmesan; toss to coat, adding pasta water if needed to loosen the sauce.
Return chicken to the pan, stir to combine, and warm through.
Finish with lemon juice and parsley before serving.
