In a large bowl, toss the shrimp with a hefty sprinkling of salt, and massage gently. Allow it to sit for 5 minutes then rinse. This helps plump the shrimp especially if they were previously frozen.
Add the Cajun seasoning to the shrimp and let it marinate for 20 minutes, while you make the chutney.
In a skillet, warm 1 tablespoon olive of oil over low heat, until shimmering Add the cumin and fennel, it should splutter.
Stir in the onion and serrano pepper and saute until softened and has picked up a little color. Add the turmeric and saute for 30 seconds. Stir in the pineapple and toss to coat with the spices. Cook for about 5 minutes. Add the cilantro and mayonnaise and stir to combine.
Add some cornmeal to a shallow plate. Coat each shrimp evenly with cornmeal, while you warm up a few tablespoons of vegetable oil in a large skillet over medium-high heat. When oil is shimmering (hot), add the shrimp in batches. Flip after 2 minutes, or when they start to change color. Cook another 2 minutes on the other side, until cooked to your liking (don't overcook, and should only take 5 minutes, tops!). Remove the shrimp from the skillet to a paper towel-lined plate.
To build the po' boy: Slather the pineapple-mayonnaise mixture on 1 side of each bun bottom. Spoon a generous helping of shrimp over the top. Add some lettuce and tomato and cover with the bun tops. Wrap in plastic and aluminum foil and you're ready for the picnic!