For the som tum: Combine the tomatoes, carrots, cucumbers and parsnips in a medium bowl. Mix together the fish sauce, brown sugar, chili flakes and lime juice in a separate bowl, tasting and adjusting the chili flakes to desired spice. Pour the dressing over the vegetables and toss to combine.
Preheat enough oil to submerge three quarters of the fish in a large wok or large straight-sided skillet to 375 degrees F.
Make 3 evenly spaced score marks on each side of the fish, taking care not to cut all the way through. Combine the tempura flour, salt and black pepper in a large shallow dish. Press each side of the fish firmly into the mixture to coat, making sure it is completely covered in the flour. Slide the whole lemongrass stalk all the way through the mouth of the fish.
Cook the fish in the oil, without flipping, until golden brown and crispy, 5 to 7 minutes. Gently spoon the hot oil over the top of the fish using a small ladle or spoon to ensure the exposed part cooks through. Transfer to a paper-towel-lined plate or a cooling rack fitted over a baking sheet.
Quickly fry the basil leaves, garlic cloves and sliced lemongrass in the hot oil until brown and crispy, just under 30 seconds. Transfer to a paper-towel-lined plate to let the oil drip off.
Plate the whole fish and drizzle a little lime juice over the top. Garnish with the fried basil, garlic and lemongrass. Spoon a little som tum salad on the side and finish with a couple dried red chiles. Serve immediately.