For the duxelles
1 oz. (2 Tbs.) unsalted butter, softened
1 Tbs. vegetable or sunflower oil
1/4 cup finely chopped shallots
1-1/2 cups finely chopped portobello mushrooms (from 4 large caps; remove the stems and gills before chopping, preferably in a food processor)
Kosher salt and freshly ground black pepper
1 Tbs. finely chopped fresh flat-leaf parsley

For the Madeira sauce
6 cups beef stock, homemade or store-bought
1 cup Madeira
Kosher salt and freshly ground black pepper
1 oz. (2 Tbs.) cold unsalted butter, diced

4 Store bought crepes

For assembly
3 lb. center-cut beef tenderloin, trimmed, side muscle removed
Kosher salt and freshly ground black pepper
1 tsp. vegetable or sunflower oil
2/3 cup chicken liver pâté, homemade or store-bought
1 lb. puff pastry, homemade or store-bought, thawed overnight in the refrigerator if frozen
1 large egg, lightly beaten
1 tsp. unsalted butter, softened