Start by washing and slicing all your vegetables, including tomatoes and onions. Cut the salmon into equal portions so they cook evenly. Let the salmon sit at room temperature for a few minutes before cooking. This helps it cook more evenly and stay tender.

Pat the salmon dry with a paper towel to remove excess moisture. Rub Cajun seasoning evenly on both sides of each fillet. Press the seasoning gently so it sticks well and forms a flavorful crust during cooking.

Heat a pan over medium-high heat and add a little oil. Place the salmon in the pan and cook for about 3–4 minutes on each side until fully cooked and slightly blackened. The outside should form a dark, crispy crust while the inside remains moist.

In a small bowl, mash the avocado until smooth. Mix in the mayonnaise to create a creamy, rich spread. This mixture adds balance to the spicy Cajun flavor and makes the sandwich extra satisfying.

Lightly toast your buns on a grill or toaster until golden. This step is important because it prevents the bread from getting soggy. It also adds a nice crunch to every bite.

Spread the avocado mixture on each bun. Place the cooked salmon on top, then layer with arugula, tomato slices, and red onions. Close the sandwich and gently press it together before serving.