Combine the ground beef, pork rinds or gluten-free bread crumbs, egg, milk, chopped onion, garlic, Worcestershire sauce, Dijon mustard, salt, and pepper in a large bowl. Mix gently until just combined, being careful not to overmix.

Divide the mixture into four equal portions and shape each into an oval patty about 1/2 inch thick.

Heat the olive oil in a large skillet over medium-high heat. Add the patties and cook for 4-5 minutes per side, until browned. Remove the patties from the skillet and set aside.

Melt the butter in the same skillet over medium heat. Add the sliced onions and cook for 8-10 minutes, stirring occasionally, until softened and caramelized.

Sprinkle the gluten-free flour over the onions and stir well to combine. Cook for 1-2 minutes to eliminate the raw flour taste.

Gradually whisk in the beef broth, ensuring no lumps remain. Stir in Worcestershire sauce and Dijon mustard, then bring the gravy to a gentle simmer, stirring occasionally, until it thickens.

Return the patties to the skillet, nestling them into the gravy. Reduce the heat to low, cover, and let everything simmer for about 10 minutes, or until the patties are fully cooked.

Taste the gravy and adjust the seasoning with salt and pepper as needed. Serve the Salisbury steaks hot, topped with the savory gravy, alongside mashed potatoes, rice, or steamed vegetables.