How to Make It
Prepare the Beef: Preheat oven to 350°F. Drain spinach well; press between paper towels to remove excess moisture. Melt butter in a medium-size straight-sided saucepan over medium. Add leeks and garlic, and cook, stirring occasionally, until softened, about 4 minutes. Add flour, and cook, whisking constantly, 3 minutes; whisk in cream until smooth. Add spinach, and cook, stirring occasionally, until mixture is very thick, about 5 minutes. Stir in cheese, lemon juice, nutmeg, 1 teaspoon of the salt, and 1⁄2 teaspoon of the pepper. Remove from heat, and let cool 10 minutes.
Meanwhile, using a sharp knife, make a horizontal cut through center of Beef, cutting to, but not through, other side. (The cut should be within 1⁄2 inch of other side.) Open top cut piece, as you would a book, and lay flat. Place between 2 sheets of plastic wrap. Using flat side of a meat mallet or back of a small heavy skillet, pound to 1⁄2-inch thickness and about a 13-inch square. Spread leek-spinach mixture evenly over Beef, leaving a 1⁄2-inch border around edges. Roll up Beef, and tie with kitchen twine, securing at 2-inch intervals. Brush evenly with olive oil; sprinkle with remaining 1 teaspoon salt and 1⁄2 teaspoon pepper.
Place Beef on a wire rack set over a rimmed baking pan. Bake in preheated oven 30 minutes. Increase oven temperature to 450°F (do not remove from oven). Bake until a meat thermometer registers 125°F, about 20 minutes. Remove from heat, and let stand 30 minutes before slicing.
Prepare the Sauce: While Beef rests, melt 1 tablespoon of the butter with olive oil in a large skillet over medium-high. Add mushrooms, and cook, stirring once or twice, until lightly browned, 5 to 6 minutes. Add shallot; cook, stirring constantly, until tender, 1 to 2 minutes. Stir in wine; cook, stirring constantly, 2 minutes. Stir in broth; reduce heat to medium, and simmer, stirring constantly, 5 minutes. Stir in mustard, rosemary, salt, and pepper, and cook 1 minute. Remove from heat, and stir in remaining 1 tablespoon butter until combined. Serve Sauce with Beef.