Boil the Pasta: Start by bringing a large pot of salted water to a rolling boil. Cook spaghetti according to package instructions until al dente, about 8-10 minutes. Reserve 1 cup of pasta water before draining.

Toast the Breadcrumbs: In a cast iron skillet, heat 2 tablespoons of olive oil over medium heat. Add breadcrumbs and cook until golden brown and crispy, about 3-5 minutes. Season lightly with salt and pepper, then remove from skillet.

Sauté the Garlic: In the same skillet, add 4 tablespoons of olive oil over medium heat. Once hot, add sliced garlic and crushed red pepper flakes. Cook for 2-3 minutes until fragrant and just golden.

Combine Pasta with Sauce: Add the drained spaghetti to the skillet with garlic. Pour in 3/4 cup of Parmesan cheese and reserved pasta water. Toss to combine, adding more pasta water as needed until a glossy sauce forms.

Serve and Garnish: Transfer the pasta to plates, topping generously with toasted breadcrumbs and remaining Parmesan cheese. Sprinkle with parsley and serve immediately.