Boil the pasta: Cook the linguine in well salted water until just al dente, 1 minute less than the package says. Save 1 cup of pasta water, then drain.
Season the shrimp: While the water heats, start the shell stock. Pat the shrimp dry and toss with the oil and 1 tbsp Cajun seasoning.
Brown the andouille: In a large, deep skillet over medium high heat, cook the sausage 4 to 5 minutes until browned. Move it to a plate, leaving the drippings behind.
Sear the shrimp: Add the shrimp in a single layer and cook 1 to 2 minutes per side until pink and just opaque. Set them aside with the sausage.
Build the base: Lower the heat to medium. Melt the butter, add the onion, bell pepper, and celery, and cook 4 to 5 minutes until soft. Stir in the garlic for 30 seconds. Sprinkle in the flour and stir constantly for 1 to 2 minutes.
Add the liquids: Slowly whisk in the stock, then the cream. Add the Cajun seasoning and cayenne, if using. Simmer 4 to 5 minutes until thick enough to coat a spoon. Turn the heat to low and stir in the Parmesan a handful at a time, then the lemon juice. Taste and adjust. Return the sausage and shrimp, add the crawfish, and simmer 2 to 3 minutes to heat through.
Toss and serve: Add the pasta and toss until every strand is coated, loosening with pasta water as needed. Top with green onions, parsley, and extra Parmesan.