In a large skillet, heat EVOO, two turns of the pan, over medium. Add the onion and cook until softened, 5 to 7 minutes. Add the garlic and stir 1 minute. Add the cream cheese; stir until melted. Add the cream and nutmeg; season with salt and pepper. Add the spinach and stir until heated through, about 3 minutes. Stir in the cheese.
In a large bowl, combine the sirloin, Worcestershire and black pepper; season with kosher salt. Form 4 large patties (thinner at the center for even cooking). Heat a grill pan or cast-iron skillet over medium-high and cook the burgers, turning once, about 8 minutes for medium-rare. Transfer the burgers to a plate. Add the stock to the skillet and cook, stirring, until reduced slightly, 2 to 3 minutes.
Divide the toast among 4 plates and top with the burgers. Drizzle each burger with the sauce from the skillet, then top with the creamed spinach and some crispy onions.