Prepare the Spicy Cajun Rub by mixing all the rub ingredients in a small bowl.
Prepare the tuna salad by rinsing the tuna and patting it dry with paper towels. Use your fingers to rub both sides of the 4 pieces of tuna with 1-2 tablespoons of the rub and set it aside. For more intense flavoring, cover and refrigerate for up to 24 hours.
Arrange the tuna steaks on a broiler pan coated with nonstick cooking spray. Spray the tops of the tuna with a light coat of the cooking spray.
Grill over medium heat, turning occasionally, or broil 6 inches under a preheated broiler for 10 minutes until the meat is browned.
Let the tuna steaks cool for 5 minutes before slicing thinly across the grain. Set aside.
Toss the lettuce with the mushrooms and tomatoes. Arrange a quarter of the mixture one each of 4 plates. Top each with some of the onion rings and a sliced tuna steak. Sprinkle with the cheese and croutons.