Fill a large pot with water, bring to a rolling boil over high heat, and season generously with salt.

Add spaghetti and cook according to package instructions until al dente, about 8–10 minutes.

While pasta cooks, heat olive oil in a large skillet over medium heat.

Add minced garlic and red pepper flakes to the skillet and sauté for about 1 minute until fragrant, ensuring garlic does not brown.

Pour in diced tomatoes, stir to combine, and let simmer for about 5 minutes.

Stir in spinach and cook for 2 minutes until wilted.

Reserve 1 cup of pasta cooking water, then drain pasta and add it to the skillet. Toss to coat, adding pasta water as needed to loosen the sauce.

Season with salt to taste, garnish with fresh basil, and serve topped with grated Parmesan if desired.