Bring a large pot of salted water to a boil. Cook pasta until al dente. Reserve 1 cup of pasta water, then drain.
While pasta cooks, heat olive oil in a wide skillet over medium-low heat.
Add garlic and cook for 30–60 seconds, stirring constantly until fragrant (do not brown).
Stir in crushed tomatoes, Italian seasoning, salt, black pepper, and red pepper flakes if using.
Simmer uncovered for 8–10 minutes, stirring occasionally.
Lower heat and slowly stir in heavy cream until smooth.
Add Parmesan cheese and stir until fully melted and silky.
Add cooked pasta and toss to coat.
Use reserved pasta water a little at a time to adjust sauce consistency.
Remove from heat, stir in butter if using, and rest for 2–3 minutes before serving.