Make the shrimp stock
Peel and devein raw shrimp.
Place shrimp in a bowl, sprinkle with Cajun seasoning, put in refrigerator.
Add shells to a large pot along with 2 quarts water.
Cover and simmer for 30 minutes to make shrimp stock.
Strain and set aside.
Make the roux
Add 1 cup canola oil and 1 cup all-purpose flour to a cast iron skillet preheated over medium heat
Whisk together oil and flour.
Continue whisking until the roux is the color of peanut butter or your desired color. Place close attention that you whisk the entire bottom surface of the skillet. Once roux starts browning, it can burn quickly in spots that aren't kept moving. Leave in skillet but remove from heat and set aside.
Make the gumbo
Cut sausage into rounds and brown in a cast iron skillet.
Once brown, remove sausage from skillet and place aside.
Peel and dice onions.
Dice celery and bell pepper.
Add diced vegetables to 8 quart stockpot along with 2 tablespoons canola oil.
Cook uncovered over medium heat, stirring frequently, for about 15 minutes until vegetables are tender and reduced in volume by approximately one half.
Add roux and sausage to pot with vegetables.
Stir in shrimp stock.
Add okra, tomatoes, salt, and garlic powder.
Simmer covered 45 minutes, stirring occasionally.
Remove seasoned shrimp from refrigerator.
Check crab for bits of shell.
Add shrimp, crab, and brandy if using, to pot. Stir.
Cover and simmer for 15 minutes or until shrimp is pink.
Serve over cooked rice.