Stuffed Beef Tenderloin with Burgundy Mushroom Sauce

Beef

3 (10-oz.) pkg. frozen chopped spinach, thawed
3 tablespoons unsalted butter
2 medium leeks, thinly sliced
1 garlic clove, minced
3 tablespoons all-purpose flour
1 cup heavy cream
1 ounce Parmigiano-Reggiano cheese, grated (about 1/4 cup)
2 teaspoons fresh lemon juice
1/4 teaspoon grated whole nutmeg
2 teaspoons kosher salt, divided
1 teaspoon black pepper, divided
1 (4-lb.) beef tenderloin, trimmed
1 tablespoon extra-virgin olive oil

Sauce

2 tablespoons unsalted butter, divided
1 tablespoon olive oil
2 (8-oz.) pkg. cremini mushrooms, quartered
1 medium shallot, minced
1/2 cup dry red wine (such as Burgundy)
1 cup beef broth
1 tablespoon Dijon mustard
1/2 teaspoon chopped fresh rosemary
1/2 teaspoon kosher salt
1/4 teaspoon black pepper

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.