Bring a large pot of water to a rolling boil. Add the salt and pasta, stirring immediately to prevent sticking. Cook according to package directions until al dente - usually 10 to 12 minutes. Before draining, scoop out 1 cup of the starchy cooking water and set aside. Drain the pasta and return it to the pot off the heat.

While the pasta cooks, add the avocado flesh, Greek yogurt, garlic, lemon zest, lemon juice, olive oil, salt, and pepper to a food processor or blender. Process until completely smooth and creamy, scraping down the sides once, about 1 minute. The sauce should be thick but pourable.

Add the avocado-yogurt sauce to the warm pasta in the pot. Pour in 1/4 cup of the reserved pasta water and toss vigorously with tongs until every piece is coated. Add more pasta water a splash at a time if needed - the sauce should cling to the pasta without pooling at the bottom.

Gently fold in the torn basil. Taste and adjust seasoning with more salt, pepper, or lemon juice as needed. Divide among bowls immediately, topping with more fresh basil and a pinch of red pepper flakes if you want some heat. Serve right away while the sauce is at its brightest green.