This is a traditional crawfish dish in New Orleans. This dish can be spicy or mild depending on the amount of red pepper flakes you use, so be careful.
Bring a large pot of lightly salted water to a boil. Add pasta and cook until al dente, 8 to 10 minutes; drain.
While the pasta cooks, melt the butter in a large, deep skillet over medium heat. Cook and stir the garlic in the butter 1 to 2 minutes; season with salt and pepper. Stir in the yellow onion and 1/2 tablespoon red pepper flakes; cook until the yellow onion becomes translucent. Add the crawfish tails; cook 2 to 3 minutes, stirring continually; transfer the crawfish to a bowl, leaving as many onions as possible in the skillet.
Pour the heavy cream into the skillet and bring to a boil; reduce heat to low, and simmer until thickened. Sprinkle the remaining 1/2 tablespoon red pepper flakes into the cream, return the crawfish to the sauce and simmer 2 to 3 minutes. Serve sauce over the drained pasta; garnish with chopped green onion.
Per Serving: 1505 calories; protein 37.6g; carbohydrates 99.8g; fat 109.8g; cholesterol 493.3mg; sodium 317.2mg.