Bring a large pot of salted water to a boil. Add the 8 oz of fettuccine pasta and cook according to package instructions until al dente (usually about 10-12 minutes). Drain the pasta, reserving about a cup of pasta water. Set aside.
While the pasta cooks, heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the diced chicken breasts, seasoning with salt and pepper. Cook for 6-8 minutes, stirring occasionally, until the chicken is cooked through and slightly golden. Remove the chicken from the skillet and set aside.
In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant. Add the fresh spinach and cook until wilted, about 2 minutes. This step infuses the sauce with garlic flavor and softens the spinach perfectly.
Pour 2 cups of heavy cream into the skillet with the garlic and spinach. Bring to a gentle simmer over medium heat. Gradually whisk in 1 cup of grated Parmesan cheese and 1 cup of shredded mozzarella cheese, stirring constantly until the cheeses melt and the sauce thickens. If the sauce becomes too thick, add a splash of the reserved pasta water to loosen it.
Return the cooked chicken to the skillet and stir to coat in the sauce. Then add the drained fettuccine pasta, tossing gently to combine everything evenly. Cook for an additional 1-2 minutes to allow the flavors to meld.
Season with additional salt and pepper if needed. Serve hot, garnished with extra Parmesan or fresh herbs if desired. This dish pairs wonderfully with a crisp green salad or garlic bread for a complete meal.