Bring a large pot of salted water to a boil. Cook pasta until al dente according to package instructions. Reserve ½ cup of the pasta water, then drain.
In a large skillet, heat olive oil over medium heat. Add garlic slices and red pepper flakes. Cook until the garlic is just golden—about 1–2 minutes. Watch closely to avoid burning.
Add the sardines (including oil, if desired) to the pan. Break them up gently with a spoon. Let them heat through and meld with the garlic oil, about 2–3 minutes.
Add the cooked pasta to the skillet. Toss to coat. Add lemon zest, lemon juice, chopped parsley, and a splash of the reserved pasta water. Toss everything until the pasta is glossy and coated.
Taste and adjust seasoning with salt and pepper.
Serve immediately, topped with extra parsley, a drizzle of olive oil, and toasted breadcrumbs or grated cheese if using.
