Combine ground beef, shallot, rosemary, Worcester sauce, salt, and pepper in a large bowl and mix until combined. Create 4 burger patties and set aside. Heat 2 tbsp extra virgin olive oil in a very large cast iron or heavy bottomed skillet. Add burger patties and cook on both sides to your desired doneness.
In the meantime, heat the Portobello Mushroom Sauce according to package instructions. Assemble the burgers with fresh lettuce leaves and onion rings. Top each burger with Portobello mushroom sauce and serve.