1 medium avocado, pitted and quartered

1. Do ahead: To make the Tomato Salsa, gently toss the tomatoes and onion, then add the remaining ingredients. Stir well to blend and refrigerate until ready to use.

2. Do ahead: To make the Citrus Vinaigrette, combine the olive oil, vinegar, and mustard in a blender. Pulse just until blended, then add the remaining ingredients. Pulse until well mixed, then refrigerate until ready to use.

3. Do ahead: To make the Chimichurri Sauce, whisk together the olive oil, vinegar, lemon juice, salt, and pepper. Stir in the remaining ingredients and mix until well blended. Cover and refrigerate until ready to use.

4. Light a charcoal barbecue or heat a gas grill to medium. Clean the grate and coat it with vegetable oil.

5. Preheat the oven to 400°F.

6. Season the fish fillets with the salt and Creole or Cajun mix.

7. Grill 3 to 4 minutes per side, depending on the thickness of the fillet. Use a wide, flexible spatula to turn the pieces over. Remove from the grill when just cooked through and no longer translucent in the center. Baste with some of the Chimichurri Sauce.

8. While the fish is cooking, lay the prepared tortillas on a baking sheet and use a fork to poke several holes in each one. Bake until lightly browned, about 4 minutes.

9. Overlap 2 tortillas to form one large, double-sized tostada. Sprinkle each tostada with a portion of the shredded cheese, then return them to the oven and bake 1 more minute, or until the cheese is melted.

10. Place each tostada on a plate.

11. Toss the lettuce with the Citrus Vinaigrette and corn and chopped bell pepper. Spoon a portion of the salad onto each of the prepared tortillas. Spoon a portion of the tomato salsa over the salads.

12. Place a piece of the mahimahi on each salad and drizzle the fish with the Chimichurri Sauce. Top each fillet with a quarter of the avocado.