Preheat the oven to 425 degrees F. Pierce the beef all over with a fork; season with salt and pepper. Heat 1 tablespoon vegetable oil and the rosemary in a large nonstick ovenproof skillet over medium-high heat. Add the beef and cook, turning, until browned, 8 minutes. Drizzle with 1 tablespoon Worcestershire sauce. Transfer to the oven; roast until a thermometer inserted into the center reaches 125 degrees F for medium rare, about 25 minutes. Transfer to a cutting board and let rest 5 minutes.
Meanwhile, pierce the potatoes all over with a fork. Microwave until tender, about 15 minutes. Heat the remaining 1/2 tablespoon vegetable oil in a saucepan over medium heat. Add the shallot and cook until softened, about 2 minutes. Sprinkle in the flour and cook, stirring, until incorporated, 5 to 6 minutes. Slowly add the milk; cook, whisking, until slightly thickened, 1 minute. Stir in the spinach, sour cream and the remaining 1 teaspoon Worcestershire sauce; season with salt and pepper. Simmer until warm, about 2 minutes.
Slice the beef. Halve the potatoes and fluff with a fork; top with the creamed spinach. Season with salt.