Bring a large pot of salted water to a rolling boil. Add the spaghetti and cook according to package instructions until al dente, usually about 8-10 minutes. Reserve 1 cup of the starchy pasta water before draining.

In a large cast iron skillet, heat 2 tablespoons of extra-virgin olive oil over medium heat. Add the French style breadcrumbs and stir frequently for about 3-5 minutes until they turn golden brown. Season with a pinch of salt and pepper, then set aside.

In the same skillet, add the remaining 4 tablespoons of olive oil. Toss in the thinly sliced garlic and optional crushed red pepper flakes. Cook while stirring constantly for about 2-3 minutes, until the garlic is fragrant and golden brown.

Add the drained spaghetti directly into the skillet with the sautéed garlic. Sprinkle in 3/4 cup of freshly grated Parmesan cheese, along with the reserved pasta water. Stir continuously for 2-3 minutes until the cheese melts and the sauce coats the spaghetti.

Remove the skillet from heat and plate the spaghetti immediately. Top with the toasted breadcrumbs, extra Parmesan, and chopped parsley.