1. Heat 1 tablespoon of the vegetable oil in a medium-size, heavy-bottomed saucepan over medium heat. Add the onion, fennel and garlic and cook until translucent, about 5 to 7 minutes. Add the prawn heads and shells and cook, stirring frequently, until shells turn bright red, an additional 5 to 7 minutes. Add tomato paste and cook, stirring frequently, for another 5 minutes. Add white wine and reduce by three-quarters, then add shellfish stock, thyme and orange zest. Bring sauce to a low boil and reduce by half.

2. Pass the sauce through a fine-mesh strainer into a clean saucepot over low heat, discarding solids. Add 2 tablespoons of the butter, and stir to emulsify. Season to taste with kosher salt.

3. Heat the remaining 2 tablespoons of oil in a large sauté pan over high heat. Season prawns on both sides with kosher salt and sear (in 2 batches if necessary) on both sides until lightly caramelized and cooked through, about 1 to 1 1/2 minutes per side. And the remaining 2 tablespoons of butter and the parsley. Stir to coat the prawns, and remove from the heat.

4. Divide prawns among 4 warmed shallow bowls. Pour one-quarter of finished shellfish sauce into each bowl, and garnish with reserved fennel fronds. Serve immediately with toasted sourdough bread. Serves 4.