For the tomato jam: Heat the olive oil in a small nonstick pot over medium-low heat and cook the tomato paste, stirring frequently, until it becomes lightly caramelized, 5 to 8 minutes.
Add the garlic and shallot and cook, stirring frequently, until soft, 3 to 4 minutes. Add the plum tomatoes and cook until they break down, 10 to 15 minutes.
Add the brown sugar, cider vinegar and soy sauce. Season with salt and pepper and cook until the mixture thickens, about 2 minutes. Let cool.
For the pickled carrots and daikon: Put the daikon and carrots in a large bowl. Set aside.
Whisk together the white vinegar, granulated sugar, salt and 3/4 cup plus 2 tablespoons water in a medium bowl. Toss and gently massage the shredded vegetables in the pickling liquid. Let sit for 10 minutes.
For the fried fish: Heat 3 inches of canola oil in a cast-iron skillet to 350 degrees F.
Combine the flour, cornstarch, baking powder and salt in a large bowl. Whisk in the cold sparkling water until you have a smooth batter.
Dip each piece of fish in the batter and make sure it's evenly coated. Fry until the fish is golden brown and cooked through, 3 to 4 minutes.
Remove the fish from the oil with a slotted spatula and drain on a wire rack. Season with additional salt while still hot.
For the assembly: Spread some of the tomato jam on the bottom halves of the bread. Layer the cabbage and pickled carrots and daikon on the bottom halves, followed by the cucumbers and jalapenos. Place the fish on top, then top with red onion, mayonnaise and watercress. (You can omit anything you don't like and add extra of anything you do.)
Serve immediately so the fish stays nice and crispy.