Preheat a large cast-iron skillet over medium-high to high. Bring the steaks to room temperature. Pat dry and season on both sides with kosher salt and black pepper.
Place the spinach in a clean kitchen towel (not terrycloth), gather the edges together and twist to wring dry. Using your fingertips, loosen, fluff and break up the spinach.
While the meat comes to room temperature and the cast-iron skillet heats up, in a separate skillet, heat 2 tbsp. oil, two turns of the pan, over medium-high. Add the garlic and swirl the pan until fragrant, 30 to 60 seconds. Add the spinach; season. Stir in the cream; season with nutmeg, if using. Bring to a bubble, reduce the heat to medium, and simmer at a low boil until thickened, about 10 minutes. Stir in the cheese.
When the cast-iron skillet is very hot, add 1 tbsp. oil, one turn of the pan. Add the steaks and cook, turning occasionally, 8 minutes. Transfer to a plate and let rest. Add the remaining 1 tbsp. oil, one turn of the pan, to the skillet. Add the mushrooms and cook, stirring occasionally, until browned, 4 to 5 minutes. Add the shallot, season, then add the sherry. Cook, stirring and scraping up the browned bits, until reduced a bit, 2 to 3 minutes. Swirl in the butter and toss in the parsley.
Pour the mushrooms with the juices over the steaks and serve with the creamed spinach. Top the spinach with the crispy onions.