Thaw the salmon if frozen, rinse and pat dry with paper towels; preheat the oven to 400°F (200°C). Grease a baking dish or rimmed baking sheet with cooking spray or a little oil.
In a bowl, combine the tomatoes, sliced onion, chopped oregano, olive oil, minced garlic, 1/4 teaspoon salt, and 1/4 teaspoon black pepper; toss to coat the vegetables.
Spread the tomato mixture in the prepared baking dish in an even layer and roast, uncovered, in the preheated oven for 15 minutes.
Remove the dish from the oven and place the salmon fillets skin-side down on top of the roasted tomatoes and onions; sprinkle the salmon with the remaining 1/4 teaspoon salt and 1/4 teaspoon black pepper.
Return the dish to the oven and bake, uncovered, for 15 to 20 minutes more, until the salmon is opaque, flakes easily with a fork, or reaches an internal temperature of 145°F (63°C).
Use a spatula to transfer the salmon to plates and spoon the roasted tomatoes and onions over the fillets before serving.