1. Make the Vinaigrette: In a small bowl, combine the horseradish, lime juice, fish sauce, brown sugar and olive oil and season with salt and pepper.
2. Make the Fried Shallots: In a large skillet, heat 1/4 inch of vegetable oil. Put the shallot rings in a colander and dust heavily with cornstarch, shaking to coat them well. Add the shallots to the hot oil in an even layer and fry over moderate heat until browned and crisp, about 4 minutes. Using a slotted spoon, transfer the fried shallots to paper towels to drain. Season lightly with salt.
3. Prepare the Salmon: Preheat the oven to 400°. Season the salmon with salt and pepper. In a large nonstick skillet, heat the oil until shimmering. Add the salmon skin side down and cook over high heat until the skin is browned and crisp, about 3 minutes. Turn the fillets and transfer the skillet to the oven. Cook for about 3 minutes, or until the salmon is just cooked.
4. Meanwhile, divide the cantaloupe among 4 shallow bowls. Drizzle with the buttermilk and season lightly with salt. Lay the salmon on the cantaloupe and spoon the vinaigrette on top. Sprinkle with the lime zest and grated horseradish. Scatter the celery leaves and fried shallots over the salmon and serve.