Place the bacon in a large deep frypan and cook over medium-high heat, turning occasionally until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate then chop. Set aside.
Season mahi mahi with garlic salt and pepper. Heat a frypan over medium heat. Cook mahi mahi until it flakes easily, 10 to 15 minutes. Remove from frypan and place each filet on a baking dish.
Preheat the grill and set the oven rack about 15cm from the heat source.
While the fish is cooking melt the butter in a separate frypan. Stir in the spring onions, garlic and spinach. Cook and stir until heated through, 2 to 3 minutes.
Stir in the chopped bacon, Tabasco sauce and evaporated milk. Bring to a low simmer then add the Cheddar cheese a little at a time until fully melted. Stir in more evaporated milk if needed to create a sauce. Divide the spinach mixture evenly over each mahi mahi fillet. Sprinkle with Parmesan cheese.
Grill in the preheated oven until the cheese has melted and is lightly browned, 3 to 5 minutes more.