Fill a large pot with water and bring it to a rolling boil. Add salt and the pasta, then return to a boil. Cook uncovered, stirring occasionally, until the pasta is just shy of al dente, approximately 6 minutes or according to the pasta's cooking requirements.
Before draining, reserve 1 cup of the starchy pasta cooking water. Drain the pasta and keep it warm.
While the pasta cooks, heat the olive oil in a large skillet over medium heat. Add the thinly sliced garlic and crushed red pepper flakes. Cook gently for about 45 seconds, stirring until fragrant without allowing the garlic to brown.
Pour ¾ cup of the reserved clam juice into the skillet. Bring to a gentle simmer and cook for 4–5 minutes, allowing the liquid to reduce slightly and develop flavor.
Stir in the marinara sauce and simmer for another 3–4 minutes, until the tomato sauce and clam juice are well combined.
Add the drained chopped clams and heat gently for 1–2 minutes. Avoid boiling them vigorously, as excessive cooking can make the clams tough.
Transfer the drained pasta directly into the skillet. Toss continuously to distribute the sauce evenly. Gradually add small splashes of reserved pasta water while tossing until the sauce becomes glossy and lightly coats the noodles.
Turn off the heat. Add the unsalted butter and chopped fresh parsley, tossing until the butter melts and blends into the sauce.
Taste and adjust with salt, freshly cracked black pepper, and a little lemon juice if desired.
Serve immediately while the pasta is warm and the sauce is silky.
