Bring a large pot of salted water to a rolling boil and cook the fettuccine or linguine until al dente according to package directions. Reserve 1 cup of pasta cooking water, then drain the pasta and set aside.

While the water heats, pat the chicken pieces dry with paper towels and season liberally with salt, black pepper, and garlic powder if using.

Heat a large deep skillet over medium-high heat and add the olive oil. When the oil shimmers, add the seasoned chicken in a single layer and sear without moving for 2 minutes to develop color, then stir and cook until cooked through and golden brown, about 4 to 6 minutes total depending on piece size. Transfer the chicken to a plate and tent loosely with foil.

Reduce the heat to medium and add the butter to the skillet. When it melts, add the minced garlic and chopped onion and cook, stirring, until fragrant and the onion is soft and translucent, about 3 minutes.

Add the sliced sun-dried tomatoes and crushed red pepper flakes if using. Stir and cook for 1 minute to release their oils and flavors.

Pour in the white wine or chicken broth and scrape up any browned bits from the bottom of the pan with a wooden spoon; simmer until the liquid reduces by about half, roughly 2 to 3 minutes.

Lower the heat to medium-low and stir in the heavy cream. Bring the cream to a gentle simmer, then add the grated Parmesan cheese in small handfuls, stirring until it melts and the sauce becomes glossy and slightly thickened, about 3 to 5 minutes. Taste and season with additional salt and black pepper as needed.

Fold the fresh spinach into the sauce in batches, allowing it to wilt between additions. Stir until the spinach is fully incorporated and the sauce is silky.

Return the seared chicken to the skillet along with any accumulated juices and add the cooked pasta. Toss gently to combine, adding reserved pasta water a few tablespoons at a time if the sauce needs thinning; you want the sauce to cling to the noodles, not sit in a pool. Add the lemon juice and toss once more to brighten flavors.

Serve immediately in shallow bowls, finish each portion with a drizzle of extra-virgin olive oil, additional grated Parmesan and fresh basil leaves. Adjust final seasoning with salt and pepper to taste.