Bring a large pot of lightly salted water to a boil. Add pasta and cook until al dente, 8 to 10 minutes; drain.
While pasta cooks, melt butter in a large, deep skillet over medium heat. Cook and stir garlic in butter for 1 to 2 minutes; season with salt and pepper. Stir in onion and 1/2 tablespoon red pepper flakes; cook until onion becomes translucent. Add crawfish tails; cook, stirring continually, 2 to 3 minutes. Transfer crawfish to a bowl, leaving as many onions as possible in the skillet.
Pour heavy cream into the skillet and bring to a boil; reduce heat to low and simmer until thickened. Sprinkle remaining 1/2 tablespoon red pepper flakes into cream, return crawfish to the sauce, and simmer 2 to 3 minutes.
Serve sauce over drained pasta; garnish with chopped green onion.