Start by grating the onion over the panko breadcrumbs in a mixing bowl, allowing them to soak together for about 5 minutes.

In the same bowl, combine the soaked breadcrumbs with the ground beef, minced garlic clove, egg, ketchup, crumbled bouillon cube, Worcestershire sauce, and dijon mustard. Mix the ingredients thoroughly until the mixture becomes pasty.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the patties to the skillet and cook for about 1 minute on each side, or until golden brown. Transfer the browned patties to a plate and set aside.

In the same skillet, add the finely chopped onion and minced garlic for your mushroom gravy. Sauté these aromatics over medium heat for about 2 minutes until the onions turn translucent.

Add the sliced mushrooms to the skillet with the sautéed onion and garlic. Cook for about 2-3 minutes until the mushrooms are golden brown and tender.

Lower the heat and add 2 tablespoons of unsalted butter. Once melted, sprinkle the flour over the mixture, stirring continuously for about 30 seconds.

Gradually pour in 2 cups of low-sodium beef broth, whisking continuously to prevent lumps from forming. Keep stirring until the gravy becomes smooth and begins to thicken, about 3-5 minutes.

Return the browned Salisbury Steak patties to the skillet, ensuring they are well immersed in the savory mushroom gravy. Cover and allow them to simmer on low heat for about 5-7 minutes.

Before serving, taste the gravy and adjust the seasoning with salt and pepper as desired. Serve over creamy mashed potatoes or buttered rice.