Rach's Individual Beef Wellingtons Are A Fave From More Than 20 Years Ago

Name: Rach's Individual Beef Wellingtons Are A Fave From More Than 20 Years Ago
Cuisine: European, British
Ingredients: 1½ pounds mushrooms; such as maitake and crimini or any mix you prefer; 6 tablespoons butter; 2 large shallots; finely chopped; 4 cloves garlic; 1 tablespoon fresh thyme; Salt and pepper; ¼ cup dry sherry; 4 center-cut filet mignon steaks; 2½ inches thick; Salt and black pepper; 1 tablespoon neutral oil or non-aerosol neutral cooking spray; 2 ounces brandy; 4 teaspoons Dijon mustard; One 14-oz package of puff pastry (Rach likes Dufour); defrosted for 2 hours in fridge before using; A small handful or scoop of flour; to roll dough; One 8-ounce terrine or slab of peppercorn mousse pate or truffle mousse pate; 1 egg and about 2 teaspoons water