For the chicken-fried steak: Sandwich the steak between 2 pieces of plastic wrap, then pound to 1/4-inch thickness and sprinkle all over with 2 teaspoons Cajun spice rub.
Place the buttermilk in a large shallow pan and put the steak in the buttermilk, turning to coat. Cover and refrigerate 8 hours or overnight.
For the sausage gravy: Heat a cast iron skillet over medium-high heat. Add the sausage and cook, stirring occasionally and breaking up the meat with the back of a wooden spoon, until cooked through, about 3 minutes. Transfer cooked meat to a bowl and leave the fat behind in the skillet. Leave the heat on.
Add the onions, butter and 1 teaspoon Cajun spice rub to the skillet and cook until the onions are translucent, 5 to 7 minutes.
Add the flour and cook, whisking to incorporate all the flour, about 3 minutes (do not let brown). Add the stock and bring to a simmer, whisking occasionally. Simmer 5 to 7 minutes.