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 frypan 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 chilli flakes; cook until the yellow onion becomes translucent. Add the yabby tails; cook 2 to 3 minutes, stirring continually; transfer the yabbies to a bowl, leaving as many onions as possible in the frypan.
Pour the heavy cream into the frypan and bring to a boil; reduce heat to low, and simmer until thickened. Sprinkle the remaining 1/2 tablespoon chilli 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 spring onion.