Pat the salmon fillets dry with paper towels to ensure a good sear. Season both sides of the fillets generously with salt and freshly cracked black pepper.
Heat the olive oil in a large skillet over medium-high heat. Place the salmon fillets in the pan, skin-side up (if applicable). Sear for 5 minutes per side, or until golden brown and cooked to your liking. Remove the salmon from the pan and set aside on a plate.
In the same skillet, melt the butter over medium heat. Add the diced onion and cook for 2 minutes until translucent. Stir in the minced garlic and sun-dried tomatoes, cooking for another minute until the garlic is fragrant.
Add the fresh baby spinach to the skillet. Stir occasionally until the spinach begins to wilt and reduce in volume, which should take about 2 minutes.
Reduce the heat to low. Pour in the heavy cream and add the dried oregano. Bring the mixture to a gentle simmer. Stir in the grated Parmesan cheese until it is completely melted and the sauce begins to thicken slightly.
Place the seared salmon fillets back into the skillet. Spoon the creamy sauce over the fish and let it simmer for another 1-2 minutes to heat through. Garnish with fresh parsley and serve immediately with lemon wedges.