Main Filling:

• 8 Tbsp. salted butter (1 stick), at room temperature
• 8 oz. cremini mushrooms, chopped
• 2 garlic cloves, finely chopped
• 1 shallot, chopped
• 1/4 tsp. crushed red pepper flakes
• 2 1/2 tsp. kosher salt
• 1 1/2 tsp. black pepper
• 1 (5-oz.) package baby spinach, coarsely chopped
• 1/2 tsp. grated lemon zest plus 2 tsp. lemon juice
• 1/4 cup seasoned breadcrumbs
• 1/4 cup grated parmesan cheese (about 1 oz.)
• 2 tsp. fresh rosemary, finely chopped
• 1 (4- to 5-lb.) whole center-cut beef tenderloin

Pan Sauce:

• 2 cups beef broth
• 1 cup dry red wine, such as a red blend or Pinot Noir
• 1 shallot, chopped
• 4 Tbsp. (1/2 stick) salted butter, cold and cubed