In a large bowl, mix together the ground beef, eggs, dry mustard, salt, garlic powder, black pepper, and finely chopped onion until well combined.

Divide the meat mixture into 8 equal portions and shape each portion into a patty.

Heat a large skillet over medium-high heat and add a tablespoon of olive oil.

Cook the patties in the skillet, in batches if necessary, browning well on one side for about 5 minutes before flipping and cooking through.

Remove the cooked patties and place them on paper towels to keep warm.

Spoon out all but 2 tablespoons of grease from the skillet.

Add the sliced mushrooms and beef broth to the skillet, scraping the bottom to incorporate any browned bits. Bring to a simmer and cook until mushrooms are softened, then stir in the sour cream.

Return the patties to the skillet, coat them in the gravy, and serve over mashed or riced cauliflower to keep the meal low-carb.