Beef wellington

Name: Beef wellington
Prep time: 40 min
Cook time: 50 min
Total time: 1 h 30 min
Cuisine: European, British
Ingredients: 500g cup mushrooms, chopped; 1/3 cup olive oil; 1.2kg Coles whole beef fillet; 100g butter; 1 onion, finely chopped; 2 garlic cloves, finely chopped; 1 teaspoon thyme leaves; 375g block frozen puff pastry, thawed; 100g sliced prosciutto, from the deli; 1 egg, lightly beaten; 1/4 cup Coles Brand traditional instant gravy powder; 1 cup boiling water; 1 teaspoon horseradish cream