Start by heating a dry skillet over medium heat. Once the pan is hot, lower the heat to medium-low and add the breadcrumbs. Let them toast gently, shaking the pan occasionally to ensure even browning and to prevent burning. When the breadcrumbs become fragrant and take on a light golden hue, remove them from the heat and set them aside.
Meanwhile, bring a large pot of water to a rolling boil. Once it’s boiling, generously salt the water. Add the pasta and cook according to the package instructions until al dente.
In a large pot or Dutch oven, heat the olive oil over medium-low heat. Once the oil is warm, add the sliced garlic and chili flakes. Stir frequently to keep the garlic moving and avoid burning—make sure the garlic doesn’t brown, as it should remain golden and aromatic.
When the pasta is done cooking, reserve 1 cup of the starchy pasta water, then drain the rest.
Add the cooked pasta to the garlic and chili oil mixture. Using tongs, toss the pasta well until each strand is coated. Gradually add the reserved pasta water a little at a time, allowing the oil and water to emulsify into a light, silky sauce that clings to the noodles.
Taste the pasta and adjust the seasoning by adding a few pinches of salt if needed—this is where properly salted pasta water makes a big difference.
Stir in the lemon juice, lemon zest, chopped parsley, toasted breadcrumbs, and a few cracks of black pepper, if desired. If you're using Parmesan cheese, fold it in at this stage.
Toss everything together until well combined, and serve immediately with extra chili flakes on the side for anyone who enjoys an extra kick of heat.