Bring a large pot of salted water to a boil.
Cook spaghetti until al dente according to package instructions.
Reserve ¼ cup of pasta cooking water, then drain the spaghetti.
Heat olive oil in a pan over medium heat.
Sauté chopped onion for 3 minutes until softened.
Add minced garlic and red pepper flakes, cooking for 30 seconds.
Stir in crushed tomatoes, salt, pepper, and Italian seasoning.
Simmer the sauce for 8 to 10 minutes, stirring occasionally.
Add the drained spaghetti and reserved pasta water to the sauce.
Toss together until the pasta is glossy and well coated.
Top with grated Parmesan and fresh herbs.
Serve immediately in bowls and enjoy warm.