Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente. Reserve 1 cup of pasta cooking water before draining.

Heat olive oil and butter in a large skillet over medium-high heat. When the butter bubbles and releases a nutty aroma, add the chicken. Sear until golden on the outside and cooked through, about 4 to 5 minutes. Remove chicken to a plate.

Lower the heat to medium and add the chopped onion to the skillet. Sauté until translucent and sweet, about 3 minutes, then add the garlic and sauté another 30 seconds until fragrant.

Pour in the chicken broth to deglaze the pan, scraping up any browned bits. Let it simmer for 1 to 2 minutes to reduce slightly.

Stir in the cream and bring to a gentle simmer; you will see the sauce begin to thicken. Add the grated Parmesan gradually, stirring until the sauce becomes silky and coats the back of a spoon. If the sauce is too thick, thin with reserved pasta water a splash at a time.

Return the cooked chicken to the skillet along with any juices, add peas or spinach if using, and toss to coat in the sauce. Let everything warm through for 1 to 2 minutes.

Taste and adjust seasoning with salt, pepper, and a squeeze of lemon.

Toss the drained pasta into the skillet, or combine them in a large bowl, tossing until every strand or tube is luxuriously coated.

Finish with a handful of chopped parsley or basil and a crack of black pepper. Serve immediately.