Boil salted water. Cook pasta until al dente.
Reserve 1 cup pasta water. Drain pasta. Set aside.
While pasta cooks, heat olive oil in large skillet over medium heat.
Add garlic. Cook 30 seconds until fragrant.
Add diced tomatoes. Stir well.
Add salt, pepper, and red pepper flakes if using.
Simmer 10 to 12 minutes until tomatoes break down and sauce thickens.
Add reserved pasta water if sauce is too thick.
Remove from heat. Stir in fresh basil.
Add pasta to sauce. Toss to coat.
Add Parmesan if using. Toss again.
Serve immediately. Garnish with basil, Parmesan, and olive oil.