Heat 1/4 cup of olive oil with 2 tablespoons of butter until the butter is melted and has stopped foaming; add patties and brown. Flip patties and brown evenly on both sides. Place browned patties in a greased 9x13 baking dish; set aside.
Next add the baby bellas and saute until golden (add more olive oil as necessary). Remove mushrooms from pan and pour over meat patties; set aside.
Saute onions until deep golden brown; add garlic and saute until fragrant. Spread over mushroom and meat patties; set aside.
Add remaining butter and flour to the pan and cook over medium heat, stirring constantly for about 1 minute. Whisk in beef broth and 1 tablespoon Worcestershire sauce and continue cooking while stirring for an additional 2 minutes and free of any lumps.
Pour gravy over meat, onion and mushrooms and cover baking dish with foil. Bake at 375 degrees for 45 minutes.