Bring a pot of salted water to a boil.

Cook the spaghetti until al dente according to the package instructions.

Before draining, reserve 1 cup of the pasta water.

Heat oil and butter in a large skillet over medium heat.

Once the butter has melted, add the diced onion and sauté for 4–5 minutes until soft and translucent.

Stir in the minced garlic and cook for 30–60 seconds until fragrant.

Sprinkle in the flour and whisk continuously for about 1 minute to cook out the raw flour taste.

Gradually pour in the chicken broth while whisking until the mixture is smooth.

Let it simmer for 2–3 minutes until it begins to thicken slightly.

Reduce the heat to low and slowly stir in the heavy cream.

Add the garlic powder, Italian seasoning, black pepper, red pepper flakes, and salt to taste.

Simmer for 3–5 minutes, stirring occasionally.

Add the freshly grated Parmesan cheese and stir in the lemon zest.

Add the lemon juice last.

Add the cooked spaghetti to the skillet and toss until every strand is coated in the creamy sauce.

Gradually add the reserved pasta water, a few tablespoons at a time, tossing continuously until the sauce becomes silky and perfectly coats the pasta.

Taste and adjust the seasoning with additional salt, black pepper, Parmesan, or lemon juice if needed.

Garnish with freshly chopped parsley, extra grated Parmesan, lemon zest, and freshly cracked black pepper.

Serve immediately while hot.