If you're not freezing the Wellingtons, chill them at least 1 hour (or overnight), then bake about 25 minutes for medium-rare.
Season beef with salt and pepper. Heat 1 tablespoon oil in a heavy skillet over high heat. Add half of beef; cook, turning, until browned all over, 2 to 3 minutes side. Transfer to a cutting board; repeat with second half. Let cool, 40 minutes. Cut each half into 4 equal pieces.
Roll out 1 sheet of puff pastry to a little larger than 16 by 12 inches. Trim edges; cut into four 8-by-6-inch squares. Place one piece of beef, cut-side down, on a pastry square; top with 2 tablespoons mousse, spreading evenly. Lightly beat 1 egg, then brush over edges of pastry, fold up corners to enclose filling, and gently press to seal. Repeat with remaining pastry, beef, and mousse. Arrange Wellingtons, seam-side up, on parchment-lined baking sheet; wrap well in plastic and then foil. Freeze up to 2 weeks.
Preheat oven to 425 degrees. Lightly beat remaining egg, then brush over frozen Wellingtons. Divide between 2 parchment-lined baking sheets. Bake about 35 minutes for medium-rare. Remove Wellingtons from baking sheet and let rest 10 minutes before serving with mushrooms.