Preheat: Set oven to 350°F (180°C). Grease a 9-inch quiche pan with olive oil.
Brown: In a skillet, sauté onion in olive oil until soft. Add ground beef and cook until browned. Drain all excess grease.
Veg: Add mushrooms to the skillet. Sauté until golden. Stir in garlic and Worcestershire sauce for 1 minute. Remove from heat.
Whisk: In a separate bowl, whisk eggs, cream, salt, and pepper.
Assemble: Place beef mixture in the pan. Top with shredded cheese and feta. Pour egg mixture over.
Bake: Bake for 35–45 minutes until set. Rest for 10 minutes before serving.