Follow the steps in order for the best result.
In a large pan, heat the olive oil and butter over medium heat. Add the crushed garlic and let it infuse the oil for a minute or so, being careful not to burn it.
Add the chicken breasts to the pan and cook on both sides until cooked through and tender. Ensure the internal temperature reaches 165°F (74°C).
Remove the cooked chicken fillets from the pan and slice them diagonally into thin pieces. Cover the sliced chicken to keep it warm while you prepare the sauce. Reserve the oil and pan drippings in the pan; these will add flavor to the sauce.
Add the chopped onion to the pan with the reserved oil and drippings. Cook, stirring occasionally, until the onion becomes soft and translucent.
Add the crumbled chicken stock cube and water to the pan. Allow the mixture to simmer, uncovered, until the liquid has reduced by half, concentrating the flavors.
Pour in the cream, add the chopped green onions, and stir in the grated cheese (either pecorino or romano) until the cheese is fully melted and the sauce is smooth and creamy.
While the sauce is simmering, bring a large pot of water to a rolling boil. If desired, add a pinch of salt to the boiling water to season the pasta.
Add the linguine pasta to the boiling water and cook according to package directions, until al dente (just tender). Once cooked, drain the pasta well.
Transfer the drained pasta to a serving dish or individual plates. Arrange the sliced chicken over the pasta, then generously spoon the creamy cheese sauce over the chicken and pasta. Serve immediately and enjoy this comforting and flavorful Creamy Chicken Linguine.