Heat the 1 Tbsp. oil in a wide pot (about 12” wide) set over medium, to medium-high heat. Add onions and garlic and cook 3 to 4 minutes. Add wine, stock, diced tomatoes, tomato paste, oregano and cayenne. Simmer 5 minutes, and then remove from the heat.
Preheat the oven to 375°F. Place the salmon on a parchment paper-lined baking sheet. Drizzle with the 2 Tbsp. oil and orange juice; season with salt and pepper. Bake fish 15 minutes, or until cooked through.
While the salmon bakes, finish ragout by setting the pot with the tomato mixture back over medium, to medium-high heat. When simmering, season with salt and pepper, add then add clams, mussels and shrimp. Cover and cook until clams and mussels just open, and the shrimp are heated through, about 3 to 4 minutes.
When cooked, set a piece of salmon into each of 8, heated shallow bowls. Divide and top salmon with ragout. Sprinkle with parsley and serve.
Note: Discard any clams or mussels that do not close when squeezed before cooking, or that do not open after cooking. Both are signs the bivalve was dead before cooking it and should not be eaten.