Add ground beef, Panko crumbs, eggs, crushed garlic, Worcestershire sauce, ketchup, balsamic vinegar, onion powder, sea salt, black pepper, and cayenne pepper (optional) to a medium bowl. Mix just until ingredients are blended.
Divide ground beef mixture into six equal portions, then form into oval-shaped ¾" thick patties.
Add beef tallow to a large skillet, then sear patties for 90 seconds per side using medium-high heat. Work in batches; add sliced onions to the last batch to soften.
Remove patties and continue to sauté onions for a few more minutes. If needed, add 1 tablespoon of butter. Once the onions begin caramelizing, add the remaining butter and flour. Mix to form a roux and cook for an additional minute.
Add wine and broth to the pan and deglaze, scraping the bottom of the pan. Add ground mustard, salt, black pepper, and browning & seasoning sauce. Simmer until thickened, approximately 5 minutes.
Add patties back to the pan and spoon some sauce over them. Cover and simmer for 10 minutes. Serve over a bed of mashed potatoes, garnished with chopped Italian parsley (optional).