Heat the oven to 400°F (204°C).
Generously season the steak with salt and pepper and let sit.
Warm a large ovenproof sauté pan in the oven for 10 minutes.
Carefully remove the pan from the oven, place it over medium-high heat, and add the olive oil, swirling to coat the bottom.
Add the steak and sear each side for 2 minutes.
Add the tomatoes (stand back when they sizzle), then transfer the pan to the oven and cook for 5 minutes.
Transfer the steak to a cutting board.
Move the pan of tomatoes to the stove-top over medium heat.
Add the garlic and cook, stirring gently, until fragrant, about 2 minutes.
Add the butter, the parsley, and 1/8 teaspoon salt and stir to melt and combine, scraping the browned bits off the bottom of the pan.
Add the spinach, stir just to coat, then divide the spinach, tomatoes, and butter sauce between two dinner plates.
Slice the steak, divide between the plates, season with salt to taste, and serve.