Preheat the oven to 425°F (220°C). Season each salmon fillet on both sides generously with salt and pepper.
In a medium ovenproof skillet over medium-high heat, heat the oil. Add the salmon flesh-side down (the side that clearly never had skin) and the rosemary sprig and sear until the fish is golden and crisp on the bottom, 4 minutes. Flip the salmon, then place the whole pan in the oven. Let roast for 8 minutes for medium-rare or longer if you prefer more well-done fish. Carefully remove the pan from the oven, remove the salmon from the pan, and set it aside, leaving the rosemary in the pan.
Place the pan over medium heat. Add the butter. Once it’s melted, add the garlic and let sizzle until fragrant, about 1 minute. Add the white wine, and let it reduce slightly, scraping up the brown bits from the bottom the pan. Add the sun-dried tomatoes and sauté for 2 minutes. Reduce the heat to low, remove and discard the rosemary sprig, add the cream, and bring it to a gentle simmer. Let simmer, stirring occasionally, until the sauce reduces to a slightly thicker consistency, 4 to 5 minutes. Add the Parmesan cheese and stir until cheese is melted, about 1 minute. Stir in the spinach, then let it wilt in the sauce.
To serve, divide the spinach and salmon between 2 dinner plates, spoon any remaining pan sauce over the salmon and spinach and enjoy.