Preheat oven to 350 degrees.
In a large skillet, heat oil over medium high heat. Add chopped onion and cook for about 5 minutes or until translucent.
Add ground beef and cook until no longer pink. Drain fat.
Add Worcestershire sauce, ketchup, mustard, garlic powder, salt and pepper. Stir to combine and cook for another 1-2 minutes. Remove from heat.
Place loaf of French bread on a cookie sheet and cut the very top off the loaf of bread. Scoop out the center of the bread leaving about a 1" border on all sides and on the bottom.
Fill the center of the loaf with the ground beef mixture. Top with the shredded cheese.
Bake for about 15 minutes or until heated through and the cheese is melted.
Top with chopped tomato.
In a small food processor, combine mayonnaise and avocado half. Pulse until smooth. Spoon into small ziploc baggie and snip off one of the corners. Drizzle over loaf of bread.